Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G26: Focus Session: Trapping of Nanoscale Biological Objects |
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Sponsoring Units: DBP Chair: W. E. Moerner, Stanford University Room: Baltimore Convention Center 323 |
Tuesday, March 14, 2006 8:00AM - 8:36AM |
G26.00001: The Anti-Brownian Electrokinetic Trap (ABEL trap) Invited Speaker: The Anti-Brownian Electrokinetic Trap (ABEL trap) provides a means to trap and manipulate individual nanoscale objects in solution. The ABEL trap works by monitoring the Brownian motion of a single fluorescent particle of interest, and then applying a feedback electric field to the solution so that the resulting electrokinetic drift exactly cancels this Brownian motion. The ABEL trap can trap objects far smaller than can be trapped by laser tweezers. I will describe experiments on individual trapped molecules of fluorescently labeled $\lambda $-DNA. Due to its non-perturbative nature, the ABEL trap allows us to perform the first detailed studies of the conformation of individual DNA molecules in their equilibrium, unstretched state. We find that each molecule has a spectrum of conformational modes, and we experimentally determine the transition rates between modes. We compare the data to Rouse and Zimm models of polymer dynamics. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G26.00002: The ac electrokinetic elongation mechanism of DNA. Christoph Walti, Andre Germishuizen, Paul Tosch, Clemens Kaminski, Giles Davies The manipulation of molecules in a controlled manner is a crucial prerequisite for the emerging field of molecular nanotechnology. AC electrokinetics provide a powerful tool for both positioning and manipulation of molecules, as well as for inducing conformational changes in DNA. We performed three-dimensional imaging measurements of fluorescently labelled DNA strands tethered to gold microelectrodes and subjected to strong ac electric fields. The observed elongation patterns are compared with previously determined fluid flow patterns and with the electric field lines obtained from numerical simulations. We demonstrate that the major contribution to the elongation of the DNA molecules stems from the ac electrokinetic torque, supplemented by a small bias force provided by the electric-field-induced fluid flow. Further, we show that the observed restricted elongation owing to the geometry of the electrode results from a sign change in the bias force. [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G26.00003: Tracking-FCS: Correlation Spectroscopy of Individual Particles Andrew Berglund, Hideo Mabuchi Recently developed methods for trapping and tracking fluorescent particles have already been exploited for monitoring free diffusion with high spatial accuracy. These achievements suggest the possibility of performing fluorescence correlation spectroscopy (FCS) on an individual molecule as it moves freely in solution. Such an approach may enable, for example, the investigation of heterogenous folding dynamics in protein molecules in their native environment. In this talk, we will discuss our own experimental and theoretical approach to this problem. We use a spatially modulated Gaussian excitation laser and lock-in detection in order to track fluorescent particles in two dimensions. We show that Ornstein-Uhlenbeck statistics are appropriate for describing the fluctuations of a tracked particle, and we apply a generalization of the familiar (free particle) FCS equations to the situation in which a tracked particle is observed for a long period of time. Based on the same statistical model, we calculate absolute limits on the accuracy with which a particle of a given brightness and diffusion coefficient can be tracked using similar methods. Our results establish a framework for interpreting fluorescence time series and determining the feasibility of near-term goals in single-particle tracking experiments. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G26.00004: Flexible polymers under spherical confinement Angelo Cacciuto, Erik Luijten We compute the free energy of confinement $\Delta F$ for a flexible self-avoiding polymer inside a spherical cavity. We find two different regimes depending on the degree of compression. For moderate confinement the free energy exhibits a power-law dependence on the diameter~$D$ of the cavity. At larger packing fraction $\phi$, however, the excluded-volume interactions between monomers dominate and the scaling law breaks down. We demonstrate that in the low density regime $\beta\Delta F$ scales as $(R_G/D)^{3/(3\nu-1)}$, where $R_G$ is the radius of gyration of the unconstrained polymer. This behavior differs from what is observed for confinement inside an infinitely long cylinder or between parallel plates, $\beta\Delta F \sim (R_G/D)^{1/\nu}$. On the basis of our results we revisit the problem of the escape through a hole of a spherically confined polymer and provide a corrected scaling prediction for the average escape time. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G26.00005: Dynamic effects in alignment of biological macromolecules for diffraction experiments. D. Starodub, U. Weierstall, K. Schmidt, R. B. Doak, P. Fromme, J. C. H. Spence Molecular alignment by means of anisotropic polarizability interaction with a laser electric field is the crucial step in the recently proposed serial diffraction of non-crystallizable proteins, where protein damage is prevented by recording the x-ray diffraction pattern from an array of identically oriented proteins passing through the intersection of x-ray and laser beams. The proteins can be delivered using a periodically triggered Rayleigh beam of doped water droplets. The behavior of a small globular protein (lysozyme), a large protein complex (ribosome) and a rodlike virus (TMV) in a water droplet and a gas damping cell at various pressures is considered. Optimum conditions for alignment in terms of laser power, pressure in a damping cell and adiabatic field switch-on are discussed. The degree of molecular alignment depends on these conditions and has to be sufficient to obtain sub-nanometer resolution in the charge density maps recovered from diffraction patterns. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G26.00006: Directed Cell Assembly with Magnetic Nanowires Edward J. Felton, Marcie Jaffee, Daniel H. Reich, Christopher S. Chen Control of the positioning and movement of mammalian cells in culture has a variety of important applications ranging from medical diagnostics to tissue engineering. We have developed cell manipulation techniques that exploit the magnetic properties of high aspect ratio nanoparticles such as ferromagnetic nanowires. The large remanent magnetization of these nanoparticles permits delivery of cells bound to them to precise locations on biochips through the nanoparticles' interactions with the magnetic fields of micropatterned magnetic structures. Together with chemical functionalization of the surfaces to direct cell adhesion, a wide range of multicellular patterns can be achieved. These approaches have been used to produce directed assemblies of cells. Results of these experiments include localized heterotypic cell pairs and the extension of these techniques to cell localization in three dimensions. [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 10:12AM |
G26.00007: Tracking Protein-coated Particles in 3D. Invited Speaker: The utilization of 2-photon microscopy in the field of Cell Biology is of increasing importance because it allows imaging of living cells, including those systems where UV imaging is not possible due to photobleaching or photodamage limitations. We propose a novel approach using 2-photon excitation based on the use of a scanner to produce an effective ``intensity trap''. As the particle moves in this trap (note that there is no force applied on the particle at the power level we are using for particle detection), the detection system continuously calculates the position of the particle in the trap. As the position of the particle is calculated with respect to the trap, the scanner position is moved to minimize the ``modulation'' of the light intensity in the trap. In practice, we set the scanner to perform an orbit around the particle in about 1 millisecond. The sampling rate is chosen such that many points (32 or 64) are acquired during the orbit. An FFT (Fast Fourier Transform) is performed on the points acquired during one orbit or after a series of orbits. The DC, AC and phase of the first harmonic of the FFT are calculated. The value of the modulation varies monotonically as the distance of the particle from the center of the orbit is increased so that for every value of the modulation we can estimate the value of the distance of the particle from the center of the orbit. The phase of the first harmonic gives the angular position of the particle with respect to the scanner zero phase which is known relative to the lab coordinates. The effective bandwidth of the tracking system depends on the maximum frequency for sinusoidal oscillation of the scanner, which is about 5 kHz for our galvano-scanner and on the number of photons needed for detecting the particle against the noise. Of course, there are other important considerations. First, if the motion of the particle is too fast such that after one orbit the particle moves too far from the new position calculated based on the previous orbit, tracking is lost since the feedback mechanism is too slow. Therefore, single molecules, which in water would move across the PSF in about 0.1 ms, cannot be tracked. We need at least a macromolecule the size of a large protein (100kD) or relatively high viscosity to increase the time a fluorescent particle can be observed in the PSF. The second consideration, perhaps the most important, is that the particle should not bleach during the length of the tracking. This is not a problem for particles made of many fluorophores, such as polystyrene fluorescent beads, which are also relatively large. Surprisingly, for relatively large particles such as viruses, photobleaching did not occur. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G26.00008: A Device of Tracking a Single Nanometer-Sized Particle in 3D with Nanometer Resolution and Millisecond Response Time. Hu Cang, C. Shan Xu, Daniel Montiel, Haw Yang We demonstrate a microscope system based on a confocal setup that can `track' a moving particle in three dimensions (3D) and `trap' it at a target position with nanometer spatial and millisecond time resolution. This is achieved by moving a 3D piezoelectric translation stage controlled by a feedback circuit to `cancel' the displacement of the particle. By keeping the target particle inside the confocal volume, it is now possible to perform other spectroscopic experiments on the particle simultaneously for a long time while the particle is moving freely in solutions. Our device overcomes the difficulty of single molecule spectroscopy (SMS) experiment on free moving samples. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G26.00009: \textbf{Nanoscale molecular traps} Chia-Fu Chou, Qihuo Wei, Jian Gu, Frederic Zenhausern, Nathan Swami We have constructed nanoscale molecular traps using electrodeless, or insulator-based, dielectrophoresis [1, 2]. The molecular traps consist an array of nanoscale dielectric constrictions defined using electron-beam lithography on nanofluidic passages. The device was then sealed using an extremely simple room-temperature sealing process with virtually no pressure applied. Upon the application of an external ac electric field, the field will be focused at the constrictions and high field gradient can be generated to trap molecules dynamically in aqueous solutions. We demonstrated the trapping of small protein molecules in an array of these nanoscale molecular traps down to 50 nm in size. [1] C.F. Chou, J.O. Tegenfeldt, O. Bakajin, S.S. Chan, E.C. Cox, N. Darnton, T.A.J. Duke, R.H. Austin (2002). ``Electrodeless Dielectrophoresis of Single and Double Stranded DNA'', \textit{Biophys. J.} \textbf{83}, 2170-2179. [2] C.F. Chou, F. Zenhausern (2003). ``Electrodeless Dielectrophoresis for Micro Total Analysis Systems'', \textit{IEEE Eng. Med. Biol.}, Nov./Dec., 62-67. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G26.00010: Internal Structure, Fluctuations and Micromechanical Properties of Bovine Arterial Endothelial Cells: An Optical Tweezers Study Carolyn Perretta, Sheena Farrell, Olga Latinovic, H. Daniel Ou-Yang The purpose of this study is to probe the micromechanical properties of cultured bovine arterial endothelial cells by using optical tweezers to trap endogenous granular structures in the cells. A novel application of oscillating optical tweezers and coherent detection of the forced oscillation of the trapped particle enables us to measure the viscoelastic properties in soft matter with a broad frequency range and with a high data sampling rate. This study was designed to determine the difference between the viscoelasticity of the cytoskeleton around granular structures in close vicinity of the nucleus and around the cell's edge. Time dependent measurements of the mechanical properties at a fixed oscillation frequency revealed pronounced fluctuation in living cells, indicating local dynamics of the cytoskeleton around the probed particle. Possible causes for the fluctuations will be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 10:48AM - 11:00AM |
G26.00011: Atomic force microscopy electrostatic nanolithography for proteins study in wiseana iridovirus and barley chromosomes Ewa Rowicka, Olga Mayevska, Sergei Lyuksyutov, Megumi Sasou, Shigeru Sugiyama Manipulation of proteins and DNA at the nanoscale has been studied using atomic force microscopy electrostatic nanolithography (AFMEN) for two different biological objects: iridovirus wiseana, and stained barley chromosomes. Partially relaxed chromosomes were characterized using scanning near field optical/atomic force microscopy based on bent-type optical probe, which was used as a cantilever for constant force AFM mode. Virus capsids and chromosomes can be treated as polarized dielectrics in strong non-uniform electric field (up to 10$^{9}$ Vm$^{-1})$ induced by biased AFM tip. It is suspected that an electric field inside a polar medium produces energy densities sufficient for either structural changes or reorganization of the protein structure. Recent results related to manipulation of proteins using AFMEN will be presented. [Preview Abstract] |
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