Friday, September 23, 2011

Digital holographic tracking of microprobes for multipoint viscosity measurements

G. Bolognesi, S. Bianchi, and R. Di Leonardo

Digital holographic microscopy provides an ideal tool for 3D tracking of microspheres while simultaneously allowing a full and accurate characterization of their main physical properties such as: radius and refractive index. We demonstrate that the combination of high resolution multipoint tracking and accurate optical sizing of tracers provides an ideal tool for precise multipoint viscosity measurements. We also report a detailed evaluation of the technique’s accuracy and precision in relation to the primary sources of error.


Static and dynamic behavior of two optically bound microparticles in a standing wave

O. Brzobohatý, V. Karásek, M. Šiler, J. Trojek, and P. Zemánek

It is generally accepted that the interaction between particles mediated by the scattered light (called optical binding) is very weak. Therefore, the optical binding is usually neglected in a multi-particle trapping in distinct optical traps. Here we show that even the presence of only two dielectric particles confined in the standing wave leads to their significantly different behavior comparing to the case of a single trapped particle. We obtained persuading coincidence between our experimental records and the results of the deterministic and stochastic theoretical simulations based on the coupled dipole method.


Optofluidic immobility of particles trapped in liquid-filled hollow-core photonic crystal fiber

M. K. Garbos, T. G. Euser, and P. St. J. Russell

We study the conditions under which a particle, laser-guided in a vertically-oriented hollow-core photonic crystal fiber filled with liquid, can be kept stationary against a microfluidic counter-flow. An immobility parameter—the fluid flow rate required to immobilize a particle against the radiation force produced by unit guided optical power—is introduced to quantify the conditions under which this occurs, including radiation, viscous and gravity forces. Measurements show that this parameter depends strongly on the ratio of particle radius a to core radius R, peaking at an intermediate value of a/R. The results follow fairly well the theoretical estimates of the optical (calculated approximately using a ray optics approach) and numerically simulated drag forces. We suggest that the system has potential applications in, e.g., measurement of the diameter, refractive index and density of particles, synthesis and biomedical research.


Tuesday, September 20, 2011

Calcium imaging in the optical stretcher

Markus Gyger, Daniel Rose, Roland Stange, Tobias Kießling, Mareike Zink, Ben Fabry, and Josef A. Käs

The Microfluidic Optical Stretcher (MOS) has previously been shown to be a versatile tool to measure mechanical properties of single suspended cells. In this study we combine optical stretching and fluorescent calcium imaging. A cell line transfected with a heat sensitive cation channel was used as a model system to show the versatility of the setup. The cells were loaded with the Ca2+ dye Fluo-4 and imaged with confocal laser scanning microscopy while being stretched. During optical stretching heat is transferred to the cell causing a pronounced Ca2+ influx through the cation channel. The technique opens new perspectives for investigating the role of Ca2+ in regulating cell mechanical behavior.


Epigenetic inheritance of elongated phenotypes between generations revealed by individual-cell-based direct observation

Yuichi Wakamoto and Kenji Yasuda

A cellular phenotype is considered to be determined not only by genetic information but also by convoluted information on past states of a cell and its ancestors, i.e. hysteresis. This 'hysteretic effect' forms the basis of epigenetic phenomena. To understand these phenomena by which cells transmit certain phenotypes to descendants, it is necessary to observe individual cells and compare the phenotypes of each between generations under stringently controlled environmental conditions. We, therefore, did an individual-cell-based differential assay using Escherichia coli as a model organism. We observed normal-sized isolated cells change into elongated phenotypes, and subsequently measured the transmission of their characteristics between generations. This change occurred when the final length of the normal cells exceeded their cell-length boundary, i.e., 10 µm with 5% probability. Once a cell became elongated, it divided unequally, producing two daughter cells; one was elongated and the other was normal. The elongated daughter transmitted the elongated phenotype to one lineage of the descendants by repeating unequal cell divisions with an average interdivision time half that of the normal phenotype, whereas the normal daughter retained normal phenotypic characteristics. The results suggest one possible non-genetic inheritance of cellular characteristics where phenotypic differences can only be inherited by geometrical information, independent of specific gene regulation.


Monday, September 19, 2011

Shaping the light transmission through a multimode optical fibre: complex transformation analysis and applications in biophotonics

Tomáš Čižmár and Kishan Dholakia

We present a powerful approach towards full understanding of laser light propagation through multimode optical fibres and control of the light at the fibre output. Transmission of light within a multimode fibre introduces randomization of laser beam amplitude, phase and polarization. We discuss the importance of each of these factors and introduce an experimental geometry allowing full analysis of the light transmission through the multimode fibre and subsequent beam-shaping using a single spatial light modulator. We show that using this approach one can generate an arbitrary output optical field within the accessible field of view and range of spatial frequencies given by fibre core diameter and numerical aperture, respectively, that contains over 80% of the total available power. We also show that this technology has applications in biophotonics. As an example, we demonstrate the manipulation of colloidal microparticles.


Optical forces and optical torques on various materials arising from optical lattices in the Lorentz-Mie regime

Lin Jia and Edwin L. Thomas

By combining the Maxwell stress tensor with the finite-difference time-domain (FDTD) method, we calculate the optical force and optical torque on particles from optical lattices. We compare our method to the two-component method and the electrostatic approximation (ESA). We also discuss how particle's refractive index, shape, size, and the morphology of an optical lattice influence optical forces and the condition to form stable optical trapping wells. In addition to optical forces, optical torque from one dimensional (1D) optical lattice is discussed for particles having anisotropic shapes; metastable and stable equilibrium orientation states are found. A detailed understanding of the optical force and torque from optical lattices has significant implications for optical trapping, micromanipulation, and sorting of particles.


Interface shear microrheometer with an optically driven oscillating probe particle

Chang Young Park, H. Daniel Ou-Yang, and Mahn Won Kim

We report the first experimental demonstration of an active interfacial shear microrheometer (ISMR) that uses a particle trapped by oscillating optical tweezers (OT) to probe the shear modulus Gs*(ω) of a gas/liquid interface. The most significant advantages of the oscillating OT in a rheology study are: (1) very high sensitivity compared to other active microrheology methods and (2) the ability to measure both the real and imaginary components of the complex shear modulus without relying on the use of Kramers-Kronig relation, which can be problematic at low frequencies for most of the passive methods. We demonstrate the utilities of our ISMR in two case studies: (1) a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine monolayer and (2) a composite of poly(styrene sulfonate) and dioctadecyldimethylammonium at the air/water interface in regimes where no other active instruments can explore.


Remodelling of Cortical Actin Where Lytic Granules Dock at Natural Killer Cell Immune Synapses Revealed by Super-Resolution Microscopy

Alice C. N. Brown, Stephane Oddos, Ian M. Dobbie, Juha-Matti Alakoskela, Richard M. Parton, Philipp Eissmann, Mark A. A. Neil, Christopher Dunsby, Paul M. W. French, Ilan Davis, Daniel M. Davis

Natural Killer (NK) cells are innate immune cells that secrete lytic granules to directly kill virus-infected or transformed cells across an immune synapse. However, a major gap in understanding this process is in establishing how lytic granules pass through the mesh of cortical actin known to underlie the NK cell membrane. Research has been hampered by the resolution of conventional light microscopy, which is too low to resolve cortical actin during lytic granule secretion. Here we use two high-resolution imaging techniques to probe the synaptic organisation of NK cell receptors and filamentous (F)-actin. A combination of optical tweezers and live cell confocal microscopy reveals that microclusters of NKG2D assemble into a ring-shaped structure at the centre of intercellular synapses, where Vav1 and Grb2 also accumulate. Within this ring-shaped organisation of NK cell proteins, lytic granules accumulate for secretion. Using 3D-structured illumination microscopy (3D-SIM) to gain super-resolution of ~100 nm, cortical actin was detected in a central region of the NK cell synapse irrespective of whether activating or inhibitory signals dominate. Strikingly, the periodicity of the cortical actin mesh increased in specific domains at the synapse when the NK cell was activated. Two-colour super-resolution imaging revealed that lytic granules docked precisely in these domains which were also proximal to where the microtubule-organising centre (MTOC) polarised. Together, these data demonstrate that remodelling of the cortical actin mesh occurs at the central region of the cytolytic NK cell immune synapse. This is likely to occur for other types of cell secretion and also emphasises the importance of emerging super-resolution imaging technology for revealing new biology.


Trapping and rotating nanoparticles using a plasmonic nano-tweezer with an integrated heat sink

Kai Wang, Ethan Schonbrun, Paul Steinvurzel & Kenneth B. Crozier

Although optical tweezers based on far-fields have proven highly successful for manipulating objects larger than the wavelength of light, they face difficulties at the nanoscale because of the diffraction-limited focused spot size. This has motivated interest in trapping particles with plasmonic nanostructures, as they enable intense fields confined to sub-wavelength dimensions. A fundamental issue with plasmonics, however, is Ohmic loss, which results in the water, in which the trapping is performed, being heated and to thermal convection. Here we demonstrate the trapping and rotation of nanoparticles using a template-stripped plasmonic nanopillar incorporating a heat sink. Our simulations predict an ~100-fold reduction in heating compared with previous designs. We further demonstrate the stable trapping of polystyrene particles, as small as 110 nm in diameter, which can be rotated around the nanopillar actively, by manual rotation of the incident linear polarization, or passively, using circularly polarized illumination.


Significant improvement of optical traps by tuning standard water immersion objectives

S Nader S Reihani, Shahid A Mir, Andrew C Richardson and Lene B Oddershede

Focused infrared lasers are widely used for micromanipulation and visualization of biological specimens. An inherent practical problem is that off-the-shelf commercial microscope objectives are designed for use with visible and not infrared wavelengths. Less aberration is introduced by water immersion objectives than by oil immersion ones, however, even water immersion objectives induce significant aberration. We present a simple method to reduce the spherical aberration induced by water immersion objectives, namely by tuning the correction collar of the objective to a value that is ~ 10% lower than the physical thickness of the coverslip. This results in marked improvements in optical trapping strengths of up to 100% laterally and 600% axially from a standard microscope objective designed for use in the visible range. The results are generally valid for any water immersion objective with any numerical aperture.


Thursday, September 15, 2011

Controlling ghost traps in holographic optical tweezers

Christina Hesseling, Mike Woerdemann, Andreas Hermerschmidt, and Cornelia Denz

Computer-generated holograms displayed by phase-modulating spatial light modulators have become a well-established tool for beam shaping purposes in holographic optical tweezers. Still, the generation of light intensity patterns with high spatial symmetry and simultaneously without interfering ghost traps is a challenge. We have implemented an iterative Fourier transform algorithm that is capable of controlling these ghost traps and demonstrate the benefit of this approach in the experiment.


Tuesday, September 13, 2011

3D lithography by rapid curing of the liquid instabilities at nanoscale

Simonetta Grilli, Sara Coppola, Veronica Vespini, Francesco Merola, Andrea Finizio, and
Pietro Ferraro

In liquids realm, surface tension and capillarity are the key forces driving the formation of the shapes pervading the nature. The steady dew drops appearing on plant leaves and spider webs result from the minimization of the overall surface energy [Zheng Y, et al. (2010) Nature 463:640–643]. Thanks to the surface tension, the interfaces of such spontaneous structures exhibit extremely good spherical shape and consequently worthy optical quality. Also nanofluidic instabilities generate a variety of fascinating liquid silhouettes, but they are however intrinsically short-lived. Here we show that such unsteady liquid structures, shaped in polymeric liquids by an electrohydrodynamic pressure, can be rapidly cured by appropriate thermal treatments. The fabrication of many solid microstructures exploitable in photonics is demonstrated, thus leading to a new concept in 3D lithography. The applicability of specific structures as optical tweezers and as novel remotely excitable quantum dots–embedded microresonators is presented.

Direct measurements of DNA-mediated colloidal interactions and their quantitative modeling

W. Benjamin Rogers and John C. Crocker

DNA bridging can be used to induce specific attractions between small particles, providing a highly versatile approach to creating unique particle-based materials having a variety of periodic structures. Surprisingly, given the fact that the thermodynamics of DNA strands in solution are completely understood, existing models for DNA-induced particle interactions are typically in error by more than an order of magnitude in strength and a factor of two in their temperature dependence. This discrepancy has stymied efforts to design the complex temperature, sequence and time-dependent interactions needed for the most interesting applications, such as materials having highly complex or multicomponent microstructures or the ability to reconfigure or self-replicate. Here we report high-spatial resolution measurements of DNA-induced interactions between pairs of polystyrene microspheres at binding strengths comparable to those used in self-assembly experiments, up to 6 kBT. We also describe a conceptually straightforward and numerically tractable model that quantitatively captures the separation dependence and temperature-dependent strength of these DNA-induced interactions, without empirical corrections. This model was equally successful when describing the more complex and practically relevant case of grafted DNA brushes with self-interactions that compete with interparticle bridge formation. Together, our findings motivate a nanomaterial design approach where unique functional structures can be found computationally and then reliably realized in experiment.


The Inverse Problem of Red Blood Cells Deformed by Optical Tweezers

X. Z. Zhou, F. P. Zhao, Z. H. Sun, H. A. Wu

Optical tweezers are widely used to study the mechanical properties of human red blood cells. This paper examines the inverse problem of computing constitutive parameters from the experimental loading-response data. Hyperelastic constitutive models are employed to characterize the stress–strain relationship of red blood cells. The large deformation and evolution of stress in red blood cells under different tensile loadings are investigated using finite element simulations. The results show that the Yeoh model provides a better characterization of human red blood cells. A nonlinear regression analysis method is presented to derive hyperelastic parameters from the experimental results. The obtained constitutive model and parameters are validated by comparing the force–displacement curves from finite element simulations and from experimental data.


Friday, September 9, 2011

Optical force on a discrete invisibility cloak in time-dependent fields

Patrick C. Chaumet, Adel Rahmani, Frédéric Zolla, André Nicolet, and Kamal Belkebir

We study, in time domain, the exchange of momentum between an electromagnetic pulse and a three-dimensional, discrete, spherical invisibility cloak. We find that a discrete cloak, initially at rest, would experience an electromagnetic force due to the pulse but would acquire zero net momentum and net displacement. On the other hand, we find that while the cloak may manage to conceal an object and shroud it from the electromagnetic forces associated with the pulse, the cloak itself can experience optomechanical stress on a scale much larger than the object would in the absence of the cloak. We also consider the effects of material dispersion and losses on the electromagnetic forces experienced by the cloak and show that they lead to a transfer of momentum from the pulse to the cloak.


Measuring integrated cellular mechanical stress response at focal adhesions by optical tweezers

François Bordeleau, Judicael Bessard, Yunlong Sheng, and Normand Marceau

The ability of cells to sustain mechanical stress is largely modulated by the cytoskeleton. We present a new application of optical tweezers to study cell's mechanical properties. We trap a fibronectin-coated bead attached to an adherent H4II-EC3 rat hepatoma cell in order to apply the force to the cell surface membrane. The bead position corresponding to the cell's local mechanical response at focal adhesions is measured with a quadrant detector. We assessed the cell response by tracking the evolution of the equilibrium force for 40 cells selected at random and selected a temporal window to assess the cell initial force expression at focal adhesions. The mean value of the force within this time window over 40 randomly selected bead/cell bounds was 52.3 pN. Then, we assessed the responses of the cells with modulation of the cytoskeletons, namely the ubiquitous actin-microfilaments and microtubules, plus the differentiation-dependent keratin intermediate filaments. Notably, a destabilization of the first two networks led to around 50 and 30% reductions in the mean equilibrium forces, respectively, relative to untreated cells, whereas a loss of the third one yielded a 25% increase. The differences in the forces from untreated and treated cells are resolved by the optical tweezers experiment.


Measurement of elastic light scattering from two optically trapped microspheres and red blood cells in a transparent medium

Matti Kinnunen, Antti Kauppila, Artashes Karmenyan, and Risto Myllylä

Optical tweezers can be used to manipulate small objects and cells. A trap can be used to fix the position of a particle during light scattering measurements. The places of two separately trapped particles can also be changed. In this Letter we present elastic light scattering measurements as a function of scattering angle when two trapped spheres are illuminated with a He–Ne laser. This setup is suitable for trapping noncharged homogeneous spheres. We also demonstrate measurement of light scattering patterns from two separately trapped red blood cells. Two different illumination schemes are used for both samples.

Tailored optical force fields using evolutionary algorithms

Colin C. Olson, Ross T. Schermer, and Frank Bucholtz

We introduce a method whereby the electromagnetic field that governs the force on a Rayleigh particle can be tailored such that the resultant force field conforms to a desired geometry. The electromagnetic field is expanded as a set of vector spherical wavefunctions (VSWFs) that describe the field over all space. Given the incident field, the resultant force on a given Rayleigh particle can be calculated throughout a volume of interest. We use an evolutionary algorithm (EA) to search the space of coefficients governing the VSWFs for those that produce the desired force field. We demonstrate how Maxwell’s equations will support an “optical tunnel” that guides particles to a trap location while at the same time preventing particles outside the tunnel from approaching the trap. This result is of interest because the field is impressed throughout the domain; that is to say, once the field is generated, no additional control is required to guide the particles.


Dynamic size tuning of multidimensional optically bound matter

O. Brzobohatý, V. Karásek, T. Čižmár, and P. Zemánek

We generate and dynamically control one-, two- and three-dimensional optically bound structures of soft matter in the geometry of counter-propagating incoherent laser beams. We report results for the Bessel, Gaussian, and Laguerre-Gaussian laser modes and particularly focus on the influence of the lateral dimensions of the beam profile on the resulting self-arranged optically bound structures. Employing the transfer of the orbital angular momentum of light in the Laguerre-Gaussian beams, we show that optically bound structures can conserve their spatial arrangements even while orbiting along the beam circumference.


Wednesday, September 7, 2011

Neuronal chemotaxis by optically manipulated liposomes

G. Pinato, L. T. Lien, E. D'Este, V. Torre, D. Cojoc

We probe chemotaxis of single neurons, induced by signalling molecules which were optically delivered from liposomes in the neighbourhood of the cells. We implemented an optical tweezers setup combined with a micro-dissection system on an inverted microscope platform. Molecules of Netrin-1 protein were encapsulated into micron-sized liposomes and manipulated to micrometric distances from a specific growth cone of a hippocampal neuron by the IR optical tweezers. The molecules were then released broken the liposomes with UV laser pulses. Chemotaxis induced by the delivered molecules was confirmed by the migration of the growth cone toward the liposome position. Since the delivery can be manipulated with high temporal and spatial resolution and the number of molecules released can be controlled quite precisely by tuning the liposome size and the solution concentration, this technique opens new opportunities to investigate the effect of physiological active compounds as Netrin-1 to neuronal signalling and guidance, which represents an important issue in neurobiology.


Noise associated with nonconservative forces in optical traps

Michel de Messieres, Natalia A. Denesyuk, and Arthur La Porta

It is known that for a particle held in an optical trap the interaction of thermal fluctuations with a nonconservative scattering force can cause a persistent nonequilibrium probability flux in the particle position. We investigate position fluctuations associated with this nonequilibrium flux analytically and through simulation. We introduce a model which reproduces the nonequilibrium effects, and in which the magnitude of additional position fluctuations can be calculated in closed form. The ratio of additional nonconservative fluctuations to direct thermal fluctuations scales inversely with the square root of trap power, and is small for typical experimental parameters. In a simulated biophysical experiment the nonconservative scattering force does not significantly increase the observed fluctuations in the length of a double-stranded DNA tether.


Nonconservative forcing and diffusion in refractive optical traps

Ingmar Saberi and Fred Gittes

Refractive optical trapping forces can be nonconservative in the vicinity of a stable equilibrium point even in the absence of radiation pressure. We discuss how nonconservative 3D force fields reduce, near an equilibrium point, to circular forcing in a plane; a simple model of such forcing is the refractive trapping of a sphere by four rays. We discuss in general the diffusion of an anisotropically trapped, circularly forced particle and obtain its spectrum of motion. Equipartition of potential energy holds, even though the nonconservative flow does not follow equipotentials of the trap. We find that the dissipated nonconservative power is proportional to temperature, providing a mechanism for runaway heating instability in traps.


Underwound DNA under Tension: Structure, Elasticity, and Sequence-Dependent Behaviors

Maxim Y. Sheinin, Scott Forth, John F. Marko, and Michelle D. Wang

DNA melting under torsion plays an important role in a wide variety of cellular processes. In the present Letter, we have investigated DNA melting at the single-molecule level using an angular optical trap. By directly measuring force, extension, torque, and angle of DNA, we determined the structural and elastic parameters of torsionally melted DNA. Our data reveal that under moderate forces, the melted DNA assumes a left-handed structure as opposed to an open bubble conformation and is highly torsionally compliant. We have also discovered that at low forces melted DNA properties are highly dependent on DNA sequence. These results provide a more comprehensive picture of the global DNA force-torque phase diagram.


Monday, September 5, 2011

Optical trapping for the characterization of amyloid-beta aggregation kinetics

Anthony J. Veloso, Hiroyuki Yoshikawa, Xin R. Cheng, Eiichi Tamiya and Kagan Kerman

Alzheimer's disease (AD) is marked by the accumulation of neuronal plaques from insoluble amyloid-beta (Aβ) peptides. Growing evidence for the role of Aβ oligomers in neuronal cell cytotoxicity and pathogenesis has prompted the development of novel techniques to better understand the early stages of aggregation. Near infrared (NIR) optical trapping was applied to characterize the early stages of Aβ aggregation in the presence of a β-sheet intercalating dye, Congo Red (CR), as the fluorescent marker. The integration of fluorescence analysis with NIR optical trapping has provided a new outlook into the first two hours of Aβ aggregation.


A single-molecule platform for investigation of interactions between G-quadruplexes and small-molecule ligands

Deepak Koirala, Soma Dhakal, Beth Ashbridge, Yuta Sannohe, Raphaël Rodriguez, Hiroshi Sugiyama, Shankar Balasubramanian & Hanbin Mao

Ligands that stabilize the formation of telomeric DNA G-quadruplexes have potential as cancer treatments, because the G-quadruplex structure cannot be extended by telomerase, an enzyme over-expressed in many cancer cells. Understanding the kinetic, thermodynamic and mechanical properties of small-molecule binding to these structures is therefore important, but classical ensemble assays are unable to measure these simultaneously. Here, we have used a laser tweezers method to investigate such interactions. With a force jump approach, we observe that pyridostatin promotes the folding of telomeric G-quadruplexes. The increased mechanical stability of pyridostatin-bound G-quadruplex permits the determination of a dissociation constant Kd of 490 ± 80 nM. The free-energy change of binding obtained from a Hess-like process provides an identical Kd forpyridostatin and a Kd of 42 ± 3 µM for a weaker ligand RR110. We anticipate that this single-molecule platform can provide detailed insights into the mechanical, kinetic and thermodynamic properties of liganded bio-macromolecules, which have biological relevance.


The photonic integration of non-solid media using optofluidics

Holger Schmidt & Aaron R. Hawkins

Photonics has long been used to study non-solid materials such as liquids, gases and plasmas, but these fluidic media have traditionally not comprised a functional part of the photonic device or system. The emerging field of optofluidics seeks to create new ways of uniting solid and non-solid materials in a single photonic system whose optical properties are typically defined by the fluidic component. This Review summarizes the current state of optofluidics from a photonics perspective. First, we describe a new class of photonic elements based on the combination of fluidic media and integrated optical structures. We then discuss the applications of optofluidic principles to particle sensing and manipulation in fluids, and finally assess current challenges and potential directions for future developments.


Friday, September 2, 2011

Towards total photonic control of complex-shaped colloids by vortex beams

Clayton P. Lapointe, Thomas G. Mason, and Ivan I. Smalyukh

We demonstrate optical trapping and orientational control over colloidal particles having complex shapes in an anisotropic host fluid using a dynamic holographic optical tweezers system. Interactions between a colloidal particle and the toroidal intensity distributions of focused Laguerre-Gaussian beams allow for stable optical tweezing and provide a tunable tilt of the particle out of the focal plane. Use of an aligned nematic liquid crystal as the host fluid suppresses rotations about the optical axis arising from angular momentum transfer from the beam and effectively defines a rotational axis for the colloid within the trap.


Optical delivery of liposome encapsulated chemical stimuli to neuronal cells

Giulietta Pinato, Tiziano Raffaelli, Elisa D’Este, Federica Tavano, and Dan Cojoc

Spatially confined and precise time delivery of neuroactive molecules is an important issue in neurophysiology. In this work we developed a technique for delivering chemical stimuli to cultured neurons consisting in encapsulating the molecules of interest in liposomes. These vectors were then loaded in reservoirs consisting of glass capillaries. The reservoirs were placed in the recording chamber and single liposomes were trapped and transported out by optical tweezers to the site of stimulation on cultured neurons. Finally, the release of liposome content was induced by application of UV-pulses, breaking the liposome membrane. The efficiency of encapsulation and release were first evaluated by loading the liposomes with fluorescein. In order to test the effect of the UV-induced release, liposomes with diameter ranging from 1 to 10 μm (fL to pL volumes), were filled with KCl and tested on neuronal cells. Neuronal cultures, loaded with Ca2+dye, were monitored by imaging intracellular Ca2+. An efficient release from the liposomes was demonstrated by detectable calcium signals, indicating stimulated depolarization of the neuronal cells by KCl. The present technique represents an alternative method for focal chemical stimulation of cultured cells that circumvents some of the limitations of microejection and photorelease of caged compounds.


Thursday, September 1, 2011

Improving Free-Energy Estimates from Unidirectional Work Measurements: Theory and Experiment

Matteo Palassini and Felix Ritort

We derive analytical expressions for the bias of the Jarzynski free-energy estimator from N nonequilibrium work measurements, for a generic work distribution. To achieve this, we map the estimator onto the random energy model in a suitable scaling limit parametrized by (log⁡N)/μ, where μ measures the width of the lower tail of the work distribution, and then compute the finite-N corrections to this limit with different approaches for different regimes of (log⁡N)/μ. We show that these expressions describe accurately the bias for a wide class of work distributions and exploit them to build an improved free-energy estimator from unidirectional work measurements. We apply the method to optical tweezers unfolding and refolding experiments on DNA hairpins of varying loop size and dissipation, displaying both near-Gaussian and non-Gaussian work distributions.