Microfluidic bio-particle manipulation for biotechnology

Barbaros Çetin, Mehmet Bülent Özer, Mehmet Ertuğrul Solmaz

Microfluidics and lab-on-a-chip technology offers unique advantages for the next generation devices for diagnostic therapeutic applications. For chemical, biological and biomedical analysis in microfluidic systems, there are some fundamental operations such as separation, focusing, filtering, concentration, trapping, detection, sorting, counting, washing, lysis of bio-particles, and PCR-like reactions. The combination of these operations led to the complete analysis systems for specific applications. Manipulation of the bio-particles is the key ingredient for these applications. Therefore, microfluidic bio-particle manipulation has attracted a significant attention from the academic community. Considering the size of the bio-particles and the throughput of the practical applications, manipulation of the bio-particles is a challenging problem. Different techniques are available for the manipulation of bio-particles in microfluidic systems. In this review, some of the techniques for the manipulation of bio-particles; namely hydrodynamic based, electrokinetic-based, acoustic-based, magnetic-based and optical-based methods have been discussed. The comparison of different techniques and the recent applications regarding the microfluidic bio-particle manipulation for different biotechnology applications are presented. Finally, challenges and the future research directions for microfluidic bio-particle manipulation are addressed.

DOI

Ultrafast polarization response of an optically-trapped single ferroelectric nanowire

Sanghee Nah , Yi-Hong Kuo , Frank Chen , Joonsuk Park , Robert Sinclair , and Aaron Lindenberg

One-dimensional potassium niobate nanowires are of interest as building blocks in integrated piezoelectric devices, exhibiting large nonlinear optical and piezoelectric responses. Here we present femtosecond measurements of light-induced polarization dynamics within an optically-trapped ferroelectric nanowire, using the second-order nonlinear susceptibility as a real-time structural probe. Large amplitude, reversible modulations of the nonlinear susceptibility are observed within single nanowires at megahertz repetition rates, developing on few-picosecond time-scales, associated with anomalous coupling of light into the nanowire.

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Reflectivity and transmissivity of a cavity coupled to a nanoparticle

M. A. Khan, K. Farooq, S. C. Hou, Shanawer Niaz, X. X. Yi

Any dielectric nanoparticle moving inside an optical cavity generates an optomechanical interaction. In this paper, we theoretically analyze the light scattering of an optomechanical cavity which strongly interacts with a dielectric nanoparticle. The cavity is driven by an external laser field. This interaction gives rise to different dynamics that can be used to cool, trap and levitate nanoparticle. We analytically calculate reflection and transmission rate of the cavity field, and study the time evolution of the intracavity field, momentum and position of the nanoparticle. We find the nanoparticle occupies a discrete position inside the cavity. This effect can be exploited to separate nanoparticle and couplings between classical particles and quantized fields.

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Sequence-resolved free energy profiles of stress-bearing vimentin intermediate filaments

Beatrice Ramm, Johannes Stiglera, Michael Hinczewski, D. Thirumalai, Harald Herrmann, Günther Woehlke, and Matthias Rief

Intermediate filaments (IFs) are key to the mechanical strength of metazoan cells. Their basic building blocks are dimeric coiled coils mediating hierarchical assembly of the full-length filaments. Here we use single-molecule force spectroscopy by optical tweezers to assess the folding and stability of coil 2B of the model IF protein vimentin. The coiled coil was unzipped from its N and C termini. When pulling from the C terminus, we observed that the coiled coil was resistant to force owing to the high stability of the C-terminal region. Pulling from the N terminus revealed that the N-terminal half is considerably less stable. The mechanical pulling assay is a unique tool to study and control seed formation and structure propagation of the coiled coil. We then used rigorous theory-based deconvolution for a model-free extraction of the energy landscape and local stability profiles. The data obtained from the two distinct pulling directions complement each other and reveal a tripartite stability of the coiled coil: a labile N-terminal half, followed by a medium stability section and a highly stable region at the far C-terminal end. The different stability regions provide important insight into the mechanics of IF assembly.

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Note: Three-dimensional linearization of optical trap position detection for precise high speed diffusion measurements

Y.-H. Hsu and A. Pralle

Studies of the details of Brownian motion, hydrodynamic of colloids, or protein diffusion measurements all require high temporal and spatial resolution of the position detector and a means to trap the colloid. Optical trap based thermal noise imaging employing a quadrant photodiode as detector provides such a method. However, optical trapping requires an objective with high numerical aperture resulting in highly nonlinear position signal and significant cross-dependence of the three spatial directions. Local diffusion measurements are especially susceptible to distance errors. Here, we present a position calibration method, which corrects nonlinearities sufficiently to allow precise local diffusion measurement throughout the entire trapping volume. This correction permits us to obtain high-resolution two- and three-dimensional diffusion maps.

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Selective particle trapping and optical binding in the evanescent field of an optical nanofiber

M. C. Frawley, I. Gusachenko, V. G. Truong, M. Sergides, and S. Nic Chormaic

The evanescent field of an optical nanofiber presents a versatile interface for the manipulation of micron-scale particles in dispersion. Here, we present a detailed study of the optical binding interactions of a pair of 3.13 μm SiO2 spheres in the nanofiber evanescent field. Preferred equilibrium positions for the spheres as a function of nanofiber diameter and sphere size are discussed. We demonstrated optical propulsion and self-arrangement of chains of one to seven 3.13 μm SiO2 particles; this effect is associated with optical binding via simulated trends of multiple scattering effects. Incorporating an optical nanofiber into an optical tweezers setup facilitated the individual and collective introduction of selected particles to the nanofiber evanescent field for experiments. Computational simulations provide insight into the dynamics behind the observed behavior.

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Measuring kinetic energy changes in the mesoscale with low acquisition rates

É. Roldán, I. A. Martínez, L. Dinis and R. A. Rica

We report on the measurement of the average kinetic energy changes in isothermal and non-isothermal quasistatic processes in the mesoscale, realized with a Brownian particle trapped with optical tweezers. Our estimation of the kinetic energy change allows to access to the full energetic description of the Brownian particle. Kinetic energy estimates are obtained from measurements of the mean square velocity of the trapped bead sampled at frequencies several orders of magnitude smaller than the momentum relaxation frequency. The velocity is tuned applying a noisy electric field that modulates the amplitude of the fluctuations of the position and velocity of the Brownian particle, whose motion is equivalent to that of a particle in a higher temperature reservoir. Additionally, we show that the dependence of the variance of the time-averaged velocity on the sampling frequency can be used to quantify properties of the electrophoretic mobility of a charged colloid. Our method could be applied to detect temperature gradients in inhomogeneous media and to characterize the complete thermodynamics of biological motors and of artificial micro and nanoscopic heat engines.

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Absolute calibration of forces in optical tweezers

R. S. Dutra, N. B. Viana, P. A. Maia Neto, and H. M. Nussenzveig

Optical tweezers are highly versatile laser traps for neutral microparticles, with fundamental applications in physics and in single molecule cell biology. Force measurements are performed by converting the stiffness response to displacement of trapped transparent microspheres, employed as force transducers. Usually, calibration is indirect, by comparison with fluid drag forces. This can lead to discrepancies by sizable factors. Progress achieved in a program aiming at absolute calibration, conducted over the past 15 years, is briefly reviewed. Here we overcome its last major obstacle, a theoretical overestimation of the peak stiffness, within the most employed range for applications, and we perform experimental validation. The discrepancy is traced to the effect of primary aberrations of the optical system, which are now included in the theory. All required experimental parameters are readily accessible. Astigmatism, the dominant effect, is measured by analyzing reflected images of the focused laser spot, adapting frequently employed video microscopy techniques. Combined with interface spherical aberration, it reveals a previously unknown window of instability for trapping. Comparison with experimental data leads to an overall agreement within error bars, with no fitting, for a broad range of microsphere radii, from the Rayleigh regime to the ray optics one, for different polarizations and trapping heights, including all commonly employed parameter domains. Besides signaling full first-principles theoretical understanding of optical tweezers operation, the results may lead to improved instrument design and control over experiments, as well as to an extended domain of applicability, allowing reliable force measurements, in principle, from femtonewtons to nanonewtons.

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Mechanical Role of Actin Dynamics in the Rheology of the Golgi Complex and in Golgi-Associated Trafficking Events

David Guet, Kalpana Mandal, Mathieu Pinot, Jessica Hoffmann, Yara Abidine, Walter Sigaut, Sabine Bardin, Kristine Schauer, Bruno Goud, Jean-Baptiste Manneville

In vitro studies have shown that physical parameters, such as membrane curvature, tension, and composition, influence the budding and fission of transport intermediates. Endocytosis in living cells also appears to be regulated by the mechanical load experienced by the plasma membrane. In contrast, how these parameters affect intracellular membrane trafficking in living cells is not known. To address this question, we investigate here the impact of a mechanical stress on the organization of the Golgi complex and on the formation of transport intermediates from the Golgi complex. Using confocal microscopy, we visualize the deformation of Rab6-positive Golgi membranes applied by an internalized microsphere trapped in optical tweezers and simultaneously measure the corresponding forces. Our results show that the force necessary to deform Golgi membranes drops when actin dynamics is altered and correlates with myosin II activity. We also show that the applied stress has a long-range effect on Golgi membranes, perturbs the dynamics of Golgi-associated actin, and induces a sharp decrease in the formation of Rab6-positive vesicles from the Golgi complex as well as tubulation of Golgi membranes. We suggest that acto-myosin contractility strongly contributes to the local rigidity of the Golgi complex and regulates the mechanics of the Golgi complex to control intracellular membrane trafficking.

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Mechanical Properties of Base-Modified DNA Are Not Strictly Determined by Base Stacking or Electrostatic Interactions

Justin P. Peters, Lauren S. Mogil, Micah J. McCauley, Mark C. Williams, L. James Maher III

This work probes the mystery of what balance of forces creates the extraordinary mechanical stiffness of DNA to bending and twisting. Here we explore the relationship between base stacking, functional group occupancy of the DNA minor and major grooves, and DNA mechanical properties. We study double-helical DNA molecules substituting either inosine for guanosine or 2,6-diaminopurine for adenine. These DNA variants, respectively, remove or add an amino group from the DNA minor groove, with corresponding changes in hydrogen-bonding and base stacking energy. Using the techniques of ligase-catalyzed cyclization kinetics, atomic force microscopy, and force spectroscopy with optical tweezers, we show that these DNA variants have bending persistence lengths within the range of values reported for sequence-dependent variation of the natural DNA bases. Comparison with seven additional DNA variants that modify the DNA major groove reveals that DNA bending stiffness is not correlated with base stacking energy or groove occupancy. Data from circular dichroism spectroscopy indicate that base analog substitution can alter DNA helical geometry, suggesting a complex relationship among base stacking, groove occupancy, helical structure, and DNA bend stiffness.

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Probing interfacial dynamics and mechanics using submerged particle microrheology. II. Experiment

Thomas Boatwright, Michael Dennin, Roie Shlomovitz, Arthur A. Evans and Alex J. Levine

A non-contact microrheological technique to probe the mechanics of the air/water interface is explored. Polystyrene spheres dissolved in water are trapped with an optical tweezer near the free surface of water, allowing the response functions of the particles to be measured as a function of the distance from the air/water interface. These measurements show that at the surface, the imaginary part of the response function increases by approximately 30% from the Stokes value measured in the bulk. As the particle is moved away from the surface via an optical trap, the response function returns to the bulk value. The method is tested by comparing the response function of particles near a rigid wall to the theory developed by Faxèn. A newly developed hydrodynamic theory is used to explain the results at the free interface through a calculation of the linear response function as a function of depth. These results show a range of sensitivity that can be utilized to study the microrheology of a Langmuir monolayer without distorting its structure.

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Simple applications of microparticle transportation by tender optical scattering force

Hideharu Kotari, Masahiro Motosuke

This paper provides a novel application of the optical radiation pressure for microfluidic particle transportation without precise focusing or alignment of the laser beam to the device and target. An optical manipulation of particles in a microfluidic platform is highly exploited in life science or biomedical analysis using optical tweezers with the use of a gradient force of the optical radiation pressure. Our method utilizes the other term of the radiation pressure, namely scattering force, to manipulate particles in a microchannel. The migration distance of particle depends on the amount of light received by the particle. Therefore, particle movement with long retention distance can be achieved by large-area irradiation even with low energy density. In our experiments, two proof-of-concept microfluidic chips were designed and investigated; one was a lateral particle sorting using a monolithic microfluidic chip integrated with a planar SU-8 waveguide and beam expander, the other was a vertical sorting using a 10-cm-long polydimethylsiloxane channel with whole-area irradiation. Experimental results show that 1, 2 and 5 μm polystyrene beads can be transported by the optical scattering force and that particle migration is achieved with the irradiated energy density <10 mW/mm2. The present method has practical potential for simple and fuss-free use of the optical radiation pressure without spot focusing or precise alignment process of the laser beam and damage to the device and sample.

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Control of Submillimeter Phase Transition by Collective Photothermal Effect

Yushi Nishimura, Keisuke Nishida, Yojiro Yamamoto, Syoji Ito, Shiho Tokonami, and Takuya Iida
Local molecular states and biological materials in small spaces ranging from the microscale to nanoscale can be modulated for medical and biological applications using the photothermal effect (PTE). However, there have been only a few reports on exploiting the collective phenomena of localized surface plasmons (LSPs) to increase the amount of light-induced heat for the control of material states and the generation of macroscopic assembled structures. Here, we clarify that microbeads covered with a vast number of Ag nanoparticles can induce a large PTE and generate a submillimeter bubble within several tens of seconds under the synergetic effect of the light-induced force and photothermal convection enhanced by collective phenomena of LSPs. Control of the phase transition induced by such a "collective photothermal effect" enables rapid assembling of macroscopic structures consisting of nanomaterials, which would be used for detection of a small amount of proteins based on light-induced heat coagulation.

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Analyzing the Movement of the Nauplius 'Artemia salina' by Optical Tracking of Plasmonic Nanoparticles

Silke R. Kirchner, Michael Fedoruk, Theobald Lohmüller, Jochen Feldmann

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A single gold nanoparticle held by an optical tweezer is used as a sensor to quantify the rhythmic motion of a Nauplius larva (Artemia salina) in a water sample. This is achieved by monitoring the time dependent displacement of the trapped nanoparticle as a consequence of the Nauplius activity. A Fourier analysis of the nanoparticle's position then yields a frequency spectrum that is characteristic to the motion of the observed species. This experiment demonstrates the capability of this method to measure and characterize the activity of small aquatic larvae without the requirement to observe them directly and to gain information about the position of the larvae with respect to the trapped particle. Overall, this approach could give an insight on the vitality of certain species found in an aquatic ecosystem and could expand the range of conventional methods for analyzing water samples.

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Dynamic diffraction-limited light-coupling of 3D-maneuvered wave-guided optical waveguides

Mark Villangca, Andrew Bañas, Darwin Palima, and Jesper Glückstad

We have previously proposed and demonstrated the targeted-light delivery capability of wave-guided optical waveguides (WOWs). As the WOWs are maneuvered in 3D space, it is important to maintain efficient light coupling through the waveguides within their operating volume. We propose the use of dynamic diffractive techniques to create diffraction-limited spots that will track and couple to the WOWs during operation. This is done by using a spatial light modulator to encode the necessary diffractive phase patterns to generate the multiple and dynamic coupling spots. The method is initially tested for a single WOW and we have experimentally demonstrated dynamic tracking and coupling for both lateral and axial displacements.

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Digital colloids: reconfigurable clusters as high information density elements

Carolyn L. Phillips, Eric Jankowski, Bhaskar Jyoti Krishnatreya, Kazem V. Edmond, Stefano Sacanna, David G. Grier, David J. Pine and Sharon C. Glotzer

Through the design and manipulation of discrete, nanoscale systems capable of encoding massive amounts of information, the basic components of computation are open to reinvention. These components will enable tagging, memory storage, and sensing in unusual environments – elementary functions crucial for soft robotics and “wet computing”. Here we show how reconfigurable clusters made of N colloidal particles bound flexibly to a central colloidal sphere have the capacity to store an amount of information that increases as O(N ln(N)). Using Brownian dynamics simulations, we predict dynamical regimes that allow for information to be written, saved, and erased. We experimentally assemble an N = 4 reconfigurable cluster from chemically synthesized colloidal building blocks, and monitor its equilibrium dynamics. We observe state switching in agreement with simulations. This cluster can store one bit of information, and represents the simplest digital colloid.

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Multifunctional Plasmonic Film for Recording Near-Field Optical Intensity

Brian J. Roxworthy, Abdul M. Bhuiya, V. V. G. Krishna Inavalli, Hao Chen, and Kimani C. Toussaint , Jr.

We demonstrate the plasmonic equivalent of photographic film for recording optical intensity in the near field. The plasmonic structure is based on gold bowtie nanoantenna arrays fabricated on SiO2 pillars. We show that it can be employed for direct laser writing of image data or recording the polarization structure of optical vector beams. Scanning electron micrographs reveal a careful sculpting of the radius of curvature and height of the triangles composing the illuminated nanoantennas, as a result of plasmonic heating, that permits spatial tunability of the resonance response of the nanoantennas without sacrificing their geometric integrity. In contrast to other memory-dedicated approaches using Au nanorods embedded in a matrix medium, plasmonic film can be used in multiple application domains. To demonstrate this functionality, we utilize the structures as plasmonic optical tweezers and show sequestering of SiO2 microparticles into optically written channels formed between exposed sections of the film. The plasmonic film offers interesting possibilities for photonic applications including optofluidic channels “without walls,” in situ tailorable biochemical sensing assays, and near-field particle manipulation and sorting.

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Microcapsule mechanics: From stability to function

Martin P. Neubauer, Melanie Poehlmann, Andreas Fery

Microcapsules are reviewed with special emphasis on the relevance of controlled mechanical properties for functional aspects. At first, assembly strategies are presented that allow control over the decisive geometrical parameters, diameter and wall thickness, which both influence the capsule's mechanical performance. As one of the most powerful approaches the layer-by-layer technique is identified. Subsequently, ensemble and, in particular, single-capsule deformation techniques are discussed. The latter generally provide more in-depth information and cover the complete range of applicable forces from smaller than pN to N. In a theory chapter, we illustrate the physics of capsule deformation. The main focus is on thin shell theory, which provides a useful approximation for many deformation scenarios. Finally, we give an overview of applications and future perspectives where the specific design of mechanical properties turns microcapsules into (multi-)functional devices, enriching especially life sciences and material sciences.

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Optical trapping of individual human immunodeficiency viruses in culture fluid reveals heterogeneity with single-molecule resolution

Yuanjie Pang, Hanna Song, Jin H. Kim, Ximiao Hou & Wei Cheng

Optical tweezers use the momentum of photons to trap and manipulate microscopic objects, contact-free, in three dimensions. Although this technique has been widely used in biology and nanotechnology to study molecular motors, biopolymers and nanostructures, its application to study viruses has been very limited, largely due to their small size. Here, using optical tweezers that can simultaneously resolve two-photon fluorescence at the single-molecule level, we show that individual HIV-1 viruses can be optically trapped and manipulated, allowing multi-parameter analysis of single virions in culture fluid under native conditions. We show that individual HIV-1 differs in the numbers of envelope glycoproteins by more than one order of magnitude, which implies substantial heterogeneity of these virions in transmission and infection at the single-particle level. Analogous to flow cytometry for cells, this fluid-based technique may allow ultrasensitive detection, multi-parameter analysis and sorting of viruses and other nanoparticles in biological fluid with single-molecule resolution.

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Equilibrium orientations of oblate spheroidal particles in single tightly focused Gaussian beams

Yongyin Cao, Wenhe Song, Weiqiang Ding, Fangkui Sun, and TongTong Zhu

Based on a hybrid discrete dipole approximation (DDA) and T-matrix method, a powerful dynamic simulation model is used to find plausible equilibrium orientation landscapes of micro- and nano-spheroids of varying size and aspect ratio. Orientation landscapes of spheroids are described in both linearly and circularly polarized Gaussian beams. It’s demonstrated that the equilibrium orientations of the prolate and oblate spheroids have different performances. Effect of beam polarization on orientation landscapes is revealed as well as new orientation of oblate spheroids. The torque efficiencies of spheroids at equilibrium are also studied as functions of tilt angle, from which the orientations of the spheroids can be affirmed. This investigation elucidates a solid background in both the function and properties of micro-and nano-spheroidal particles trapped in optical tweezers.

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Speckle optical tweezers: micromanipulation with random light fields

Giorgio Volpe, Lisa Kurz, Agnese Callegari, Giovanni Volpe, and Sylvain Gigan

Current optical manipulation techniques rely on carefully engineered setups and samples. Although similar conditions are routinely met in research laboratories, it is still a challenge to manipulate microparticles when the environment is not well controlled and known a priori, since optical imperfections and scattering limit the applicability of this technique to real-life situations, such as in biomedical or microfluidic applications. Nonetheless, scattering of coherent light by disordered structures gives rise to speckles, random diffraction patterns with well-defined statistical properties. Here, we experimentally demonstrate how speckle fields can become a versatile tool to efficiently perform fundamental optical manipulation tasks such as trapping, guiding and sorting. We anticipate that the simplicity of these “speckle optical tweezers” will greatly broaden the perspectives of optical manipulation for real-life applications.

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Volatility and Oxidative Ageing of Aqueous Maleic Acid Aerosol Droplets and the Dependence on Relative Humidity

Benjamin J. Dennis-Smither , Frances H. Marshall , Rachael E.H. Miles , Thomas C Preston, and Jonathan Philip Reid

The microphysical structure and heterogeneous oxidation by ozone of single aerosol particles containing maleic acid (MA) has been studied using aerosol optical tweezers and cavity enhanced Raman spectroscopy. The evaporation rate of MA from aqueous droplets has been measured over a range of relative humidities and the pure component vapour pressure determined to be (1.7± 0.2)×10^-3 Pa. Variation in the refractive index (RI) of an aqueous MA droplet with relative humidity (RH) allowed the sub-cooled liquid RI of MA to be estimated as 1.481 ± 0.001. Measurements of the hygroscopic growth are shown to be consistent with equilibrium model predictions from previous studies. Simultaneous measurements of the droplet composition, size and refractive index have been made during ozonolysis at RHs in the range 50 to 80 %, providing insight into the volatility of organic products, changes in the droplet hygroscopicity and optical properties. Exposure of the aqueous droplets to ozone leads to the formation of products with a wide range of volatilities spanning from involatile to volatile. Reactive uptake coefficients show a weak dependence on ozone concentration, but no dependence on RH or salt concentration. The time evolving RI depends significantly on the RH at which the oxidation proceeds and can even show opposing trends; while the RI increases with ozone exposure at low relative humidity, the RI decreases when the oxidation proceeds at high relative humidity. The variations in RI are broadly consistent with a framework for predicting RIs for organic components published by Cappa et al.1 Once oxidised, particles are shown to form amorphous phases on drying rather than crystallisation, with slow evaporation kinetics of residual water.

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Unravelling the effects of radiation forces in water

Nelson G. C. Astrath, Luis C. Malacarne, Mauro L. Baesso, Gustavo V. B. Lukasievicz & Stephen E. Bialkowski

The effect of radiation forces at the interface between dielectric materials has been a long-standing debate for over a century. Yet there has been so far only limited experimental verification in complete accordance with the theory. Here we measure the surface deformation at the air–water interface induced by continuous and pulsed laser excitation and match this to rigorous theory of radiation forces. We demonstrate that the experimental results are quantitatively described by the numerical calculations of radiation forces. The Helmholtz force is used for the surface radiation pressure. The resulting surface pressure obtained is consistent with the momentum conservation using the Minkowski momentum density expression assuming that the averaged momentum per photon is given by the Minkowski momentum. Considering the total momentum as a sum of that propagating with the electromagnetic wave and that deposited locally in the material, the Abraham momentum interpretation also appears to be appropriate.

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Plasmofluidic single-molecule surface-enhanced Raman scattering from dynamic assembly of plasmonic nanoparticles

Partha Pratim Patra, Rohit Chikkaraddy, Ravi P. N. Tripathi, Arindam Dasgupta & G. V. Pavan Kumar

Single-molecule surface-enhanced Raman scattering (SM-SERS) is one of the vital applications of plasmonic nanoparticles. The SM-SERS sensitivity critically depends on plasmonic hot-spots created at the vicinity of such nanoparticles. In conventional fluid-phase SM-SERS experiments, plasmonic hot-spots are facilitated by chemical aggregation of nanoparticles. Such aggregation is usually irreversible, and hence, nanoparticles cannot be re-dispersed in the fluid for further use. Here, we show how to combine SM-SERS with plasmon polariton-assisted, reversible assembly of plasmonic nanoparticles at an unstructured metal–fluid interface. One of the unique features of our method is that we use a single evanescent-wave optical excitation for nanoparticle assembly, manipulation and SM-SERS measurements. Furthermore, by utilizing dual excitation of plasmons at metal–fluid interface, we create interacting assemblies of metal nanoparticles, which may be further harnessed in dynamic lithography of dispersed nanostructures. Our work will have implications in realizing optically addressable, plasmofluidic, single-molecule detection platforms.

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Left-handed optical radiation torque

Davit Hakobyan & Etienne Brasselet

Optical forces and torques are two mechanical degrees of freedom available to manipulate matter, and form the basis of optical tweezing strategies. In contrast to the Keplerian intuition that objects should be pushed downstream an incident photon flux, the concept of ‘negative’ optical forces has recently been described and has triggered many developments. Here, we report on the counterintuitive angular analogue of negative optical forces by demonstrating that circularly polarized Gaussian light beams give rise to torque with opposite sign to that of the incident optical angular momentum. Such a ‘left-handed’ mechanical effect is demonstrated by the use of an inhomogeneous and anisotropic transparent macroscopic medium. Practical difficulties associated with the direct observation of optically induced spinning of a macroscopic object are circumvented via the rotational Doppler effect. These results shed light on spin–orbit optomechanics and equip the left-handed optomechanical toolbox with angular features.

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Manipulation of microparticles using Bessel beams from semiconductor lasers

G. S. Sokolovskii, S. N. Losev, K. K. Soboleva, V. V. Dudelev, A. G. Deryagin, W. Sibbett, V. I. Kuchinskii, E. U. Rafailov

Optical manipulation of microscopic objects (including living cells) using Bessel beams from semiconductor lasers has been demonstrated for the first time. In addition, it has been found in the experiments that a Bessel beam of sufficient power from a semiconductor laser makes it possible to manipulate simultaneously several microscopic objects captured into its central lobe and the first ring.

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Optical manipulation of single molecules in the living cell

Kamilla Norregaard, Liselotte Jauffred, Kirstine Berg-Sørensenb and Lene B. Oddershede

Optical tweezers are the only nano-tools capable of manipulating and performing force-measurements on individual molecules and organelles within the living cell without performing destructive penetration through the cell wall and without the need for inserting a non-endogenous probe. Here, we describe how optical tweezers are used to manipulate individual molecules and perform accurate force and distance measurements within the complex cytoplasm of the living cell. Optical tweezers can grab individual molecules or organelles, if their optical contrast to the medium is large enough, as is the case, e.g., for lipid granules or chromosomes. However, often the molecule of interest is specifically attached to a handle manipulated by the optical trap. The most commonly used handles, their insertion into the cytoplasm, and the relevant micro-rheology of the cell are discussed here and we also review recent and exciting results achieved through optical force manipulation of individual molecules in vivo.

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Real-time 3D particle manipulation visualized using volume holographic gratings

Zhi Chen, Wensheng Chen, Hsin-yu Lu, Yves Chevallier, Nanguang Chen, George Barbastathis, and Yuan Luo

Holographic optical tweezers (HOTs) extend optical trapping into three dimensions. Volume imaging then becomes a concern as trapped objects are easily moved out of focus of the imaging objective lens. Here we demonstrate a three-dimensional real-time interactive optical trapping, manipulating, and imaging system based on HOTs incorporated with volume holographic microscope. Intensity information about the trapped objects at multiple depths can be captured in a single measurement. This method is compatible with most imaging modes such as bright-field and fluorescence.

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Stochastic interactions of two Brownian hard spheres in the presence of depletants

Mehdi Karzar-Jeddi, Remco Tuinier, Takashi Taniguchi and Tai-Hsi Fan

A quantitative analysis is presented for the stochastic interactions of a pair of Brownian hard spheres in non-adsorbing polymer solutions. The hard spheres are hypothetically trapped by optical tweezers and allowed for random motion near the trapped positions. The investigation focuses on the long-time correlated Brownian motion. The mobility tensor altered by the polymer depletion effect is computed by the boundary integral method, and the corresponding random displacement is determined by the fluctuation-dissipation theorem. From our computations it follows that the presence of depletion layers around the hard spheres has a significant effect on the hydrodynamic interactions and particle dynamics as compared to pure solvent and uniform polymer solution cases. The probability distribution functions of random walks of the two interacting hard spheres that are trapped clearly shift due to the polymer depletion effect. The results show that the reduction of the viscosity in the depletion layers around the spheres and the entropic force due to the overlapping of depletion zones have a significant influence on the correlated Brownian interactions.

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The dynamics of giant unilamellar vesicle oxidation probed by morphological transitions

Shalene Sankhagowit, Shao-Hua Wu, Roshni Biswas, Carson T. Riche, Michelle L. Povinelli, Noah Malmstadt

We have studied the dynamics of Lissamine Rhodamine B dye sensitization-induced oxidation of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) giant unilamellar vesicles (GUVs), where the progression of the underlying chemical processes was followed via vesicle membrane area changes. The surface-area-to-volume ratio of our spherical GUVs increased after as little as ten seconds of irradiation. The membrane area expansion was coupled with high amplitude fluctuations not typical of GUVs in isoosmotic conditions. To accurately measure the area of deformed and fluctuating membranes, we utilized a dual-beam optical trap (DBOT) to stretch GUV membranes into a geometrically regular shape. Further oxidation led to vesicle contraction, and the GUVs became tense, with micron-scale pores forming in the bilayer. We analyzed the GUV morphological behaviors as two consecutive rate-limiting steps. We also considered the effects of altering DOPC and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(lissamine rhodamine B sulfonyl) (RhDPPE) concentrations. The resulting kinetic model allows us to measure how lipid molecular area changes during oxidation, as well as to determine the rate constants controlling how quickly oxidation products are formed. Controlled membrane oxidation and permeabilization are also a potential tool for drug delivery based on engineered photosensitizer-containing lipid vesicles.

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Simultaneous trapping of two types of particles by using a focused partially coherent cosine-Gaussian-correlated Schell-model beam

M L Luo and D M Zhao

We numerically study the radiation forces (RFs) of the cosine-Gaussian-correlated Schell-model (CGSM) beam, in which the classic Gaussian degree of coherence is modulated by the cosine function, exerted on a Rayleigh dielectric sphere. By performing calculations, we found that the CGSM beam presents an advantage over the conventional partially coherent cosine-Gaussian beam in which the cosine function is employed for modeling of the spectral density, as the former can simultaneously trap and manipulate two types of particles with different refractive indices, while the latter just traps one kind of particle with high-refractive index, even under the same condition.

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Liposome-Based Liquid Handling Platform Featuring Addition, Mixing, and Aliquoting of Femtoliter Volumes

Hideaki Shiomi, Soichiro Tsuda, Hiroaki Suzuki, Tetsuya Yomo

This paper describes the utilization of giant unilamellar vesicles (GUVs) as a platform for handling chemical and biochemical reagents. GUVs with diameters of 5 to 10 µm and containing chemical/biochemical reagents together with inert polymers were fused with electric pulses (electrofusion). After reagent mixing, the fused GUVs spontaneously deformed to a budding shape, separating the mixed solution into sub-volumes. We utilized a microfluidic channel and optical tweezers to select GUVs of interest, bring them into contact, and fuse them together to mix and aliquot the reaction product. We also show that, by lowering the ambient temperature close to the phase transition temperature Tm of the lipid used, daughter GUVs completely detached (fission). This process performs all the liquid-handing features used in bench-top biochemistry using the GUV, which could be advantageous for the membrane-related biochemical assays.

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Trapping of a single DNA molecule using nanoplasmonic structures for biosensor applications

Jung-Dae Kim and Yong-Gu Lee

Conventional optical trapping using a tightly focused beam is not suitable for trapping particles that are smaller than the diffraction limit because of the increasing need of the incident laser power that could produce permanent thermal damages. One of the current solutions to this problem is to intensify the local field enhancement by using nanoplasmonic structures without increasing the laser power. Nanoplasmonic tweezers have been used for various small molecules but there is no known report of trapping a single DNA molecule. In this paper, we present the trapping of a single DNA molecule using a nanohole created on a gold substrate. Furthermore, we show that the DNA of different lengths can be differentiated through the measurement of scattering signals leading to possible new DNA sensor applications.

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Probing the micro-rheological properties of aerosol particles using optical tweezers

Rory M Power and Jonathan P Reid

The use of optical trapping techniques to manipulate probe particles for performing micro-rheological measurements on a surrounding fluid is well-established. Here, we review recent advances made in the use of optical trapping to probe the rheological properties of trapped particles themselves. In particular, we review observations of the continuous transition from liquid to solid-like viscosity of sub-picolitre supersaturated solution aerosol droplets using optical trapping techniques. Direct measurements of the viscosity of the particle bulk are derived from the damped oscillations in shape following coalescence of two particles, a consequence of the interplay between viscous and surface forces and the capillary driven relaxation of the approximately spheroidal composite particle. Holographic optical tweezers provide a facile method for the manipulation of arrays of particles allowing coalescence to be controllably induced between two micron-sized aerosol particles. The optical forces, while sufficiently strong to confine the composite particle, are several orders of magnitude weaker than the capillary forces driving relaxation. Light, elastically back-scattered by the particle, is recorded with sub-100 ns resolution allowing measurements of fast relaxation (low viscosity) dynamics, while the brightfield image can be used to monitor the shape relaxation extending to times in excess of 1000 s. For the slowest relaxation dynamics studied (particles with the highest viscosity) the presence and line shape of whispering gallery modes in the cavity enhanced Raman spectrum can be used to infer the relaxation time while serving the dual purpose of allowing the droplet size and refractive index to be measured with accuracies of ±0.025% and ±0.1%, respectively. The time constant for the damped relaxation can be used to infer the bulk viscosity, spanning from the dilute solution limit to a value approaching that of a glass, typically considered to be >1012 Pa s, whilst the frequencies of the normal modes of the oscillations of the particle can be used to infer surface properties. We will review the use of optical tweezers for studying the viscosity of aerosol particles and discuss the potential use of this micro-rheological tool for probing the fundamental concepts of phase, thermodynamic equilibrium and metastability.

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Self-assembly of skyrmion-dressed chiral nematic colloids with tangential anchoring

M. B. Pandey, T. Porenta, J. Brewer, A. Burkart, S. Čopar, S. Žumer, and Ivan I. Smalyukh

We describe dipolar nematic colloids comprising mutually bound solid microspheres, three-dimensional skyrmions, and point defects in a molecular alignment field of chiral nematic liquid crystals. Nonlinear optical imaging and numerical modeling based on minimization of Landau–de Gennes free energy reveal that the particle-induced skyrmions resemble torons and hopfions, while matching surface boundary conditions at the interfaces of liquid crystal and colloidal spheres. Laser tweezers and videomicroscopy reveal that the skyrmion-colloidal hybrids exhibit purely repulsive elastic pair interactions in the case of parallel dipoles and an unexpected reversal of interaction forces from repulsive to attractive as the center-to-center distance decreases for antiparallel dipoles. The ensuing elastic self-assembly gives rise to colloidal chains of antiparallel dipoles with particles entangled by skyrmions.

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Rotational Doppler velocimetry to probe the angular velocity of spinning microparticles

D. B. Phillips, M. P. Lee, F. C. Speirits, S. M. Barnett, S. H. Simpson, M. P. J. Lavery, M. J. Padgett, and G. M. Gibson

Laser Doppler velocimetry is a technique used to measure linear velocity, ranging from that of exhaust gases to blood flow. A rotational analog of laser Doppler velocimetry was recently demonstrated, using a rotationally symmetric interference pattern to probe the angular velocity of a spinning object. In this work, we demonstrate the use of a diffraction-limited structured illumination pattern to measure the angular velocity of a micron-sized particle trapped and spinning at tens of Hz in an optical trap. The technique requires no detailed knowledge of the shape of the particle, or the distribution of scatterers within it, and is independent of the particle's chirality, transparency, and birefringence. The particle is also subjected to Brownian motion, which complicates the signal by affecting the rotation rate and the rotation axis. By careful consideration of these influences, we show how the measurement is robust to both, representing a technique with which to probe the rotational motion of microscale particles.

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Rapid feedback control and stabilization of an optical tweezers with a budget microcontroller

Daniel Nino, Haowei Wang and Joshua N Milstein

Laboratories ranging the scientific disciplines employ feedback control to regulate variables within their experiments, from the flow of liquids within a microfluidic device to the temperature within a cell incubator. We have built an inexpensive, yet fast and rapidly deployed, feedback control system that is straightforward and flexible to implement from a commercially available Arduino Due microcontroller. This is in comparison with the complex, time-consuming and often expensive electronics that are commonly implemented. As an example of its utility, we apply our feedback controller to the task of stabilizing the main trapping laser of an optical tweezers. The feedback controller, which is inexpensive yet fast and rapidly deployed, was implemented from hacking an open source Arduino Due microcontroller. Our microcontroller based feedback system can stabilize the laser intensity to a few tenths of a per cent at 200 kHz, which is an order of magnitude better than the laserʼs base specifications, illustrating the utility of these devices.

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Calibration of optical tweezers based on an autoregressive model

Zi-Qiang Wang, Jin-Hua Zhou, Min-Cheng Zhong, Di Li, and Yin-Mei Li

The power spectrum density (PSD) has long been explored for calibrating optical tweezers stiffness. Fast Fourier Transform (FFT) based spectral estimator is typically used. This approach requires a relatively longer data acquisition time to achieve adequate spectral resolution. In this paper, an autoregressive (AR) model is proposed to obtain the Spectrum Density using a limited number of samples. According to our method, the arithmetic model has been established with burg arithmetic, and the final prediction error criterion has been used to select the most appropriate order of the AR model, the power spectrum density has been estimated based the AR model. Then, the optical tweezers stiffness has been determined with the simple calculation from the power spectrum. Since only a small number of samples are used, the data acquisition time is significantly reduced and real-time stiffness calibration becomes feasible. To test this calibration method, we study the variation of the trap stiffness as a function of the parameters of the data length and the trapping depth. Both of the simulation and experiment results have showed that the presented method returns precise results and outperforms the conventional FFT method when using a limited number of samples.

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Tube length-assisted optimized aerosol trapping

S. Mohammad-Reza Taheri, Mohammad Sadeghi, Ebrahim Madadi, S. Nader S. Reihani

Trapping a single aerosol using optical tweezers could be of great importance for environmental sciences. Though a single nanoparticle as small as 10 nm is successfully trapped in aqueous media using optical tweezers, due to spherical aberration only large clusters of nanoparticles were stably trapped in air. In this paper we provide our theoretical and experimental results on optimized trapping of aerosols as small as 400 nm in radius by the introduction of an extra spherical aberration source in order to minimize the total spherical aberration of the system. Our method allows for trapping of high refractive index particles such as polystyrene beads in air. It also provides considerably large trappable depth range which endows in-depth trapping. Our theoretical and experimental results are in very good agreement.

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In Vitro-Reconstituted Nucleoids Can Block Mitochondrial DNA Replication and Transcription

Géraldine Farge, Majda Mehmedovic, Marian Baclayon, Siet M.J.L. van den Wildenberg, Wouter H. Roos, Claes M. Gustafsson, Gijs J.L. Wuite, Maria Falkenberg

The mechanisms regulating the number of active copies of mtDNA are still unclear. A mammalian cell typically contains 1,000–10,000 copies of mtDNA, which are packaged into nucleoprotein complexes termed nucleoids. The main protein component of these structures is mitochondrial transcription factor A (TFAM). Here, we reconstitute nucleoid-like particles in vitro and demonstrate that small changes in TFAM levels dramatically impact the fraction of DNA molecules available for transcription and DNA replication. Compaction by TFAM is highly cooperative, and at physiological ratios of TFAM to DNA, there are large variations in compaction, from fully compacted nucleoids to naked DNA. In compacted nucleoids, TFAM forms stable protein filaments on DNA that block melting and prevent progression of the replication and transcription machineries. Based on our observations, we suggest that small variations in the TFAM-to-mtDNA ratio may be used to regulate mitochondrial gene transcription and DNA replication.

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Optically Trapped Bacteria Pairs Reveal Discrete Motile Response to Control Aggregation upon Cell–Cell Approach

Maria Dienerowitz, Laura V. Cowan, Graham M. Gibson, Rebecca Hay, Miles J. Padgett, Vernon R. Phoenix

Aggregation of bacteria plays a key role in the formation of many biofilms. The critical first step is cell–cell approach, and yet the ability of bacteria to control the likelihood of aggregation during this primary phase is unknown. Here, we use optical tweezers to measure the force between isolated Bacillus subtilis cells during approach. As we move the bacteria towards each other, cell motility (bacterial swimming) initiates the generation of repulsive forces at bacterial separations of ~3 μm. Moreover, the motile response displays spatial sensitivity with greater cell–cell repulsion evident as inter-bacterial distances decrease. To examine the environmental influence on the inter-bacterial forces, we perform the experiment with bacteria suspended in Tryptic Soy Broth, NaCl solution and deionised water. Our experiments demonstrate that repulsive forces are strongest in systems that inhibit biofilm formation (Tryptic Soy Broth), while attractive forces are weak and rare, even in systems where biofilms develop (NaCl solution). These results reveal that bacteria are able to control the likelihood of aggregation during the approach phase through a discretely modulated motile response. Clearly, the force-generating motility we observe during approach promotes biofilm prevention, rather than biofilm formation.

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Realization of optical pulling forces using chirality

Kun Ding, Jack Ng, Lei Zhou, and C. T. Chan

The optical force acting on a chiral particle is qualitatively different from that acting on an achiral particle due to chirality-dependent forces which couple mechanical linear momentum with optical spin angular momentum. We show that such chirality-induced coupling can serve as a mechanism to realize optical pulling forces that can be predicted analytically and are also observed in full wave simulations for chiral structures.

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A study about multi-trapping of a tapered-tip single fiber optical tweezers

Liang Pei-Bo, Lei Jiao-Jie, Liu Zhi-Hai, Zhang Yu, Yuan Li-Bo

We develop a pair of tapered-tip single fiber optical tweezers, and study its multi-trapping characteristic. The finite difference time domain (FDTD) method is employed to simulate the trapping force characteristic of this pair of single fiber optical tweezers, and the results show that the number of the trapped particles depends on the refractive index and the size of the particles. The trapping force of this pair of tapered-tip single fiber optical tweezers is calibrated by the experimental method, and the experimental results are consistent with the theoretical calculation results. The multi-trapping capability realized by the tapered-tip single fiber optical tweezers will be practical and useful for some applications in biomedical researching fields.

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Molecular population dynamics of DNA structures in a bcl-2 promoter sequence is regulated by small molecules and the transcription factor hnRNP LL

Yunxi Cui, Deepak Koirala, HyunJin Kang, Soma Dhakal, Philip Yangyuoru, Laurence H. Hurley and Hanbin Mao

Minute difference in free energy change of unfolding among structures in an oligonucleotide sequence can lead to a complex population equilibrium, which is rather challenging for ensemble techniques to decipher. Herein, we introduce a new method, molecular population dynamics (MPD), to describe the intricate equilibrium among non-B deoxyribonucleic acid (DNA) structures. Using mechanical unfolding in laser tweezers, we identified six DNA species in a cytosine (C)-rich bcl-2 promoter sequence. Population patterns of these species with and without a small molecule (IMC-76 or IMC-48) or the transcription factor hnRNP LL are compared to reveal the MPD of different species. With a pattern recognition algorithm, we found that IMC-48 and hnRNP LL share 80% similarity in stabilizing i-motifs with 60 s incubation. In contrast, IMC-76 demonstrates an opposite behavior, preferring flexible DNA hairpins. With 120–180 s incubation, IMC-48 and hnRNP LL destabilize i-motifs, which has been previously proposed to activate bcl-2 transcriptions. These results provide strong support, from the population equilibrium perspective, that small molecules and hnRNP LL can modulate bcl-2 transcription through interaction with i-motifs. The excellent agreement with biochemical results firmly validates the MPD analyses, which, we expect, can be widely applicable to investigate complex equilibrium of biomacromolecules.

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Magnetically responsive gourd-shaped colloidal particles in cholesteric liquid crystals

Bohdan Senyuk, Michael C. M. Varney, Javier A. Lopez, Sijia Wang, Ning Wu and Ivan I. Smalyukh

Particle shape and medium chirality are two key features recently used to control anisotropic colloidal self-assembly and dynamics in liquid crystals. Here, we study magnetically responsive gourd-shaped colloidal particles dispersed in cholesteric liquid crystals with periodicity comparable or smaller than the particle's dimensions. Using magnetic manipulation and optical tweezers, which allow one to position colloids near the confining walls, we measured the elastic repulsive interactions of these particles with confining surfaces and found that separation-dependent particle–wall interaction force is a non-monotonic function of separation and shows oscillatory behavior. We show that gourd-shaped particles in cholesterics reside not on a single sedimentation level, but on multiple long-lived metastable levels separated by a distance comparable to cholesteric periodicity. Finally, we demonstrate three-dimensional laser tweezers assisted assembly of gourd-shaped particles taking advantage of both orientational order and twist periodicity of cholesterics, potentially allowing new forms of orientationally and positionally ordered colloidal organization in these media.

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Inward and outward membrane tubes pulled from giant vesicles

Raktim Dasgupta and Rumiana Dimova

Membrane nanotubes are extruded from giant unilamellar lipid vesicles using a controlled hydrodynamic flow and membrane-attached beads manipulated via optical tweezers. Within a single experiment, the technique can be used to assess various important mechanical and rheological characteristics of the membrane such as the bending rigidity, tension and intermonolayer slip. The application of small flow velocities leads to the extrusion of tubes with sufficiently large diameters conveniently measurable under an optical microscope. For the first time, we show that by suitably controlling the medium flow, inward tubes inside the vesicles can be formed. This approach offers great potential for studying tubulation mechanisms in membrane systems, exhibiting positive as well as negative spontaneous curvatures and should offer a more realistic model for biomembranes because the vesicle membrane tension can adapt freely.

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Controlled 3D rotation of biological cells using optical multiple-force clamps

Yoshio Tanaka and Shin-ich Wakida

Controlled three-dimensional (3D) rotation of arbitrarily shaped objects in the observation space of optical microscopes is essential for realizing tomographic microscope imaging and offers great flexibility as a noncontact micromanipulation tool for biomedical applications. Herein, we present 3D rotational control of inhomogeneous biological samples using 3D optical multiple-force clamps based on time-shared scanning with a fast focus-tunable lens. For inhomogeneous samples with shape and optical anisotropy, we choose diatoms and their fragments, and demonstrate interactive and controlled 3D rotation about arbitrary axes in 3D Cartesian coordinates. We also outline the hardware setup and 3D rotation method for our demonstrations.

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Evidence for an electrostatic mechanism of force generation by the bacteriophage T4 DNA packaging motor

Amy D. Migliori, Nicholas Keller, Tanfis I. Alam, Marthandan Mahalingam, Venigalla B. Rao, Gaurav Arya & Douglas E. Smith

How viral packaging motors generate enormous forces to translocate DNA into viral capsids remains unknown. Recent structural studies of the bacteriophage T4 packaging motor have led to a proposed mechanism wherein the gp17 motor protein translocates DNA by transitioning between extended and compact states, orchestrated by electrostatic interactions between complimentarily charged residues across the interface between the N- and C-terminal subdomains. Here we show that site-directed alterations in these residues cause force dependent impairments of motor function including lower translocation velocity, lower stall force and higher frequency of pauses and slips. We further show that the measured impairments correlate with computed changes in free-energy differences between the two states. These findings support the proposed structural mechanism and further suggest an energy landscape model of motor activity that couples the free-energy profile of motor conformational states with that of the ATP hydrolysis cycle.

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Repulsive DNA-DNA Interactions Accelerate Viral DNA Packaging in Phage Phi29

Nicholas Keller, Damian delToro, Shelley Grimes, Paul J. Jardine, and Douglas E. Smith

We use optical tweezers to study the effect of attractive versus repulsive DNA-DNA interactions on motor-driven viral packaging. Screening of repulsive interactions accelerates packaging, but induction of attractive interactions by spermidine3+ causes heterogeneous dynamics. Acceleration is observed in a fraction of complexes, but most exhibit slowing and stalling, suggesting that attractive interactions promote nonequilibrium DNA conformations that impede the motor. Thus, repulsive interactions facilitate packaging despite increasing the energy of the theoretical optimum spooled DNA conformation.

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Selective transport control on molecular velcro made from intrinsically disordered proteins

Kai D. Schleicher, Simon L. Dettmer, Larisa E. Kapinos, Stefano Pagliara, Ulrich F. Keyser, Sylvia Jeney & Roderick Y. H. Lim

The selectivity and speed of many biological transport processes transpire from a ‘reduction of dimensionality’ that confines diffusion to one or two dimensions instead of three. This behaviour remains highly sought after on polymeric surfaces as a means to expedite diffusional search processes in molecular engineered systems. Here, we have reconstituted the two-dimensional diffusion of colloidal particles on a molecular brush surface. The surface is composed of phenylalanine-glycine nucleoporins (FG Nups)—intrinsically disordered proteins that facilitate selective transport through nuclear pore complexes in eukaryotic cells. Local and ensemble-level experiments involving optical trapping using a photonic force microscope and particle tracking by video microscopy, respectively, reveal that 1-µm-sized colloidal particles bearing nuclear transport receptors called karyopherins can exhibit behaviour that varies from highly localized to unhindered two-dimensional diffusion. Particle diffusivity is controlled by varying the amount of free karyopherins in solution, which modulates the multivalency of Kap-binding sites within the molecular brush. We conclude that the FG Nups resemble stimuli-responsive molecular ‘velcro’, which can impart ‘reduction of dimensionality’ as a means of biomimetic transport control in artificial environments.

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Single-Molecule Mechanochemical Sensing Using DNA Origami Nanostructures

Dr. Deepak Koirala, Prakash Shrestha, Tomoko Emura, Kumi Hidaka, Shankar Mandal, Prof. Dr. Masayuki Endo, Prof. Dr. Hiroshi Sugiyama, and Prof. Dr. Hanbin Mao

While single-molecule sensing offers the ultimate detection limit, its throughput is often restricted as sensing events are carried out one at a time in most cases. 2D and 3D DNA origami nanostructures are used as expanded single-molecule platforms in a new mechanochemical sensing strategy. As a proof of concept, six sensing probes are incorporated in a 7-tile DNA origami nanoassembly, wherein binding of a target molecule to any of these probes leads to mechanochemical rearrangement of the origami nanostructure, which is monitored in real time by optical tweezers. Using these platforms, 10 pm platelet-derived growth factor (PDGF) are detected within 10 minutes, while demonstrating multiplex sensing of the PDGF and a target DNA in the same solution. By tapping into the rapid development of versatile DNA origami nanostructures, this mechanochemical platform is anticipated to offer a long sought solution for single-molecule sensing with improved throughput.

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Hydrodynamic Slip on DNA Observed by Optical Tweezers-Controlled Translocation Experiments with Solid-State and Lipid-Coated Nanopores

Lukas Galla, Andreas J. Meyer, Andre Spiering, Andy Sischka, Michael Mayer, Adam R. Hall, Peter Reimann, and Dario Anselmetti

We use optical tweezers to investigate the threading force on a single dsDNA molecule inside silicon-nitride nanopores between 6 and 70 nm in diameter, as well as lipid-coated solid-state nanopores. We observe a strong increase of the threading force for decreasing nanopore size that can be attributed to a significant reduction in the electroosmotic flow (EOF), which opposes the electrophoresis. Additionally, we show that the EOF can also be reduced by coating the nanopore wall with an electrically neutral lipid bilayer, resulting in an 85% increase in threading force. All experimental findings can be described by a quantitative theoretical model that incorporates a hydrodynamic slip effect on the DNA surface with a slip length of 0.5 nm.

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To construct a stable and tunable optical trap in the focal region of a high numerical aperture lens

Gokulakrishnan Kandasamy; Suresh Ponnan; T. V. Sivasubramonia Pillai; Rajesh K. Balasundaram

Based on the diffraction theory, the focusing properties of a radially polarized quadratic Bessel–Gaussian beam (QBG) with on-axis radial phase variance wavefront are investigated theoretically in the focal region of a high numerical aperture (NA) objective lens. The phase wavefront C and pupil beam parameter μ of QBG are the functions of the radial coordinate. The detailed numerical calculation of the focusing property of a QBG beam is presented. The numerical calculation shows that the beam parameter μ and phase parameter C have greater effect on the total electric field intensity distribution. It is observed that under the condition of different μ, evolution principle of focal pattern differs very remarkably on increasing C. Also, some different focal shapes may appear, including rhombic shape, quadrangular shape, two-spherical crust focus shape, two-peak shape, one dark hollow focus, two dark hollow focuses pattern, and triangle dark hollow focus, which find wide optical applications such as optical trapping and nanopatterning.

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Phononic-Crystal-Based Acoustic Sieve for Tunable Manipulations of Particles by a Highly Localized Radiation Force

Fei Li, Feiyan Cai, Zhengyou Liu, Long Meng, Ming Qian, Chen Wang, Qian Cheng, Menglu Qian, Xin Liu, Junru Wu, Jiangyu Li, and Hairong Zheng

The ability to manipulate microscale and nanoscale particles is highly desirable for various applications ranging from targeting drug delivery to additive manufacturing. Here we report an acoustic sieve that is capable of aligning, trapping, sorting, and transferring a large number of particles according to their size or mass density, all in a tunable manner. The concept is based on the highly localized periodic radiation force induced by the resonance transmission of an acoustic wave across a phononic crystal plate, a phenomenon analogous to the surface-phonon-enhanced optical force, yet the physical concept has not been explored in acoustics. The acoustic sieve demonstrates the effective manipulation of massive particles using an artificially engineered acoustic field by a phononic crystal, and it has potential application for a wide range of applications.

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Optimization of particle trapping and patterning via photovoltaic tweezers: role of light modulation and particle size

J Matarrubia, A Garc?a-Caba?es, J L Plaza, F Agull?-L?pez and M Carrascosa
The role of light modulation m and particle size on the morphology and spatial resolution of nano-particle patterns obtained by photovoltaic tweezers on Fe?:?LiNbO3 has been investigated. The impact of m when using spherical as well as non-spherical (anisotropic) nano-particles deposited on the sample surface has been elucidated. Light modulation is a key parameter determining the particle profile contrast that is optimum for spherical particles and high-m values (m?~?1). The minimum particle periodicities reachable are also investigated obtaining periodic patterns up to 3.5??m. This is a value at least one order of magnitude shorter than those obtained in previous reported experiments. Results are successfully explained and discussed in light of the previous reported models for photorefraction including nonlinear carrier transport and dielectrophoretic trapping. From the results, a number of rules for particle patterning optimization are derived.

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Single beam optical vortex tweezers with tunable orbital angular momentum

Mindaugas Gecevičius, Rokas Drevinskas, Martynas Beresna and Peter G. Kazansky

We propose a single beam method for generating optical vortices with tunable optical angular momentum without altering the intensity distribution. With the initial polarization state varying from linear to circular, we gradually control the torque transferred to the trapped non-absorbing and non-birefringent silica beads. The continuous transition from the maximum rotation speed to zero without changing the trapping potential gives a way to study the complex tribological interactions.

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