Thursday, October 23, 2014

Pairwise Interactions of Colloids in Two-dimensional Geometric Confinement

Bum Jun Park, Bomsock Lee and Taekyung Yu

We present the pairwise interaction behaviour of colloids confined in two-dimensional (2D) colloidal cages using optical laser tweezers. A single probe particle inside the hexagonal cage particles at a planar oil-water interface is allowed to diffuse freely and the spring constant is extracted from its trajectories. To evaluate the effect of multibody interactions, the pair interactions between the probe particle and each cage particle are directly measured by using optical tweezers. Based on pairwise additivity, Monte Carlo simulations are used to compare the values of the spring constant obtained from experiments and simulations. We find that the multibody interactions negligibly occur, and thus the particle interactions confined in such colloidal cages are highly pairwise. This work demonstrates that the use of the pairwise assumption in numerical simulations is rational when interparticle repulsive interactions are sufficiently strong, such as the particle interactions at fluid-fluid interfaces.


Stability and interaction forces of oil-in-water emulsions as observed by optical tweezers – a proof-of-concept study

Julie Nilsen-Nygaard, Marit Sletmoen and Kurt Ingar Draget

Increased insight into the interactions occurring between emulsion droplets is important to a range of applications from the food and pharmaceutical industries to oil recovery and mineral flotation. These interactions are often modified by the adsorption at the oil–water interface of surface-active species such as small molecule surfactants, proteins or polymers, in order to meet functional requirements of the emulsions. However, the experimental challenges faced when attempting to study these forces acting between emulsion droplets have hampered the progress in the understanding of the fundamental forces and to which extent these forces influence the destabilizing processes. In this paper we describe emulsion droplet studies by applying optical tweezers. By capturing two emulsion droplets in separate optical traps and bringing them into proximity, the forces acting between them can be measured as a function of separation distance. In this proof-of-concept study the force versus distance curves of emulsion droplets of different stabilization was obtained. Focus has been placed on the relative differences between micro- and macromolecular stabilization of emulsion droplets. Effects on depletion interaction, relaxation behaviour of the interfacial polymer layer during compression of the droplets and electrostatic screening have been observed. The present article documents the suitability of optical tweezers in studies aiming at revealing the forces acting between individual emulsion droplets as well as limiting factors of the technology.


Chemical characterization of single micro- and nano-particles by optical catapulting–optical trapping–laser-induced breakdown spectroscopy

Francisco J. Fortes, Angel Fernández-Bravo, J. Javier Laserna

Spectral identification of individual micro- and nano-sized particles by the sequential intervention of optical catapulting, optical trapping and laser-induced breakdown spectroscopy is presented. The three techniques are used for different purposes. Optical catapulting (OC) serves to put the particulate material under inspection in aerosol form. Optical trapping (OT) permits the isolation and manipulation of individual particles from the aerosol, which are subsequently analyzed by laser-induced breakdown spectroscopy (LIBS). Once catapulted, the dynamics of particle trapping depends both on the laser beam characteristics (power and intensity gradient) and on the particle properties (size, mass and shape). Particles are stably trapped in air at atmospheric pressure and can be conveniently manipulated for a precise positioning for LIBS analysis. The spectra acquired from the individually trapped particles permit a straightforward identification of the material inspected. Variability of LIBS signal for the inspection of Ni microspheres was 30% relative standard deviation. OC–OT–LIBS permits the separation of particles in a heterogeneous mixture and the subsequent analysis of the isolated particle of interest. In order to evaluate the sensitivity of the approach, the number of absolute photons emitted by a single trapped particle was calculated. The limit of detection (LOD) for Al2O3 particles was calculated to be 200 attograms aluminium.


Determining Intrachain Diffusion Coefficients for Biopolymer Dynamics from Single-Molecule Force Spectroscopy Measurements

Michael T. Woodside, John Lambert, Kevin S.D. Beach

The conformational diffusion coefficient for intrachain motions in biopolymers, D, sets the timescale for structural dynamics. Recently, force spectroscopy has been applied to determine D both for unfolded proteins and for the folding transitions in proteins and nucleic acids. However, interpretation of the results remains unsettled. We investigated how instrumental effects arising from the force probes used in the measurement can affect the value of D recovered via force spectroscopy. We compared estimates of D for the folding of DNA hairpins found from measurements of rates and energy landscapes made using optical tweezers with estimates obtained from the same single-molecule trajectories via the transition path time. The apparent D obtained from the rates was much lower than the result found from the same data using transition time analysis, reflecting the effects of the mechanical properties of the force probe. Deconvolution of the finite compliance effects on the measurement allowed the intrinsic value to be recovered. These results were supported by Brownian dynamics simulations of the effects of force-probe compliance and bead size.


Graded-index optical fiber tweezers with long manipulation length

Yuan Gong, Wei Huang, Qun-Feng Liu, Yu Wu, Yunjiang Rao, Gang-Ding Peng, Jinyi Lang, and Ke Zhang
Long manipulation length is critical for optical fiber tweezers to enhance the flexibility of non-contact trapping. In this paper a long manipulation distance of more than 40 μm is demonstrated experimentally by the graded-index fiber (GIF) tweezers, which is fabricated by chemically etching a GIF taper with a large cone angle of 58°. The long manipulation distance is obtained by introducing an air cavity between the lead-in single mode fiber and the GIF as well as by adjusting the laser power in the existence of a constant background flow. The influence of the cavity length and the GIF length on the light distribution and the focusing length of the GIF taper is investigated numerically, which is helpful for optimizing the parameters to perform stable optical trapping. This kind of optical fiber tweezers has advantages including low-cost, easy-to-fabricate and easy-to-use.


Non-contact fiber-optical trapping of motile bacteria: dynamics observation and energy estimation

Hongbao Xin, Qingyuan Liu & Baojun Li

The dynamics and energy conversion of bacteria are strongly associated with bacterial activities, such as survival, spreading of bacterial diseases and their pathogenesis. Although different discoveries have been reported on trapped bacteria (i.e. immobilized bacteria), the investigation on the dynamics and energy conversion of motile bacteria in the process of trapping is highly desirable. Here, we report a non-contact optical trapping of motile bacteria using a modified tapered optical fiber. Using Escherichia coli as an example, both single and multiple motile bacteria have been trapped and manipulated in a non-contact manner. Bacterial dynamics has been observed and bacterial energy has been estimated in the trapping process. This non-contact optical trapping provides a new opportunity for better understanding the bacterial dynamics and energy conversion at the single cell level.


Gradient and scattering forces in photoinduced force microscopy

Junghoon Jahng, Jordan Brocious, Dmitry A. Fishman, Fei Huang, Xiaowei Li, Venkata Ananth Tamma, H. Kumar Wickramasinghe, and Eric Olaf Potma

A theoretical and experimental analysis of the dominant forces measured in photoinduced force microscopy is presented. It is shown that when operated in the noncontact and soft-contact modes, the microscope is sensitive to the optically induced gradient force (Fg) and the scattering force (Fsc). The reconstructed force-distance curve reveals a tip-dependent scattering force in the 30–60 pN range. Whereas the scattering force is virtually insensitive to the nanoscopic tip-sample distance, the gradient force shows a z−4 dependence and is manifest only for tip-sample distances of a few nm. Measurements on glass, gold nanowires, and molecular clusters of silicon naphtalocyanine confirm that the gradient force is strongly dependent on the polarizability of the sample, enabling spectroscopic imaging through force detection. The nearly constant Fsc and the spatially dependent Fg give rise to a complex force-distance curve, which varies from point to point in the specimen and dictates the image contrast observed for a given set point of the cantilevered tip.


Direct Quantification of Loop Interaction and π–π Stacking for G-Quadruplex Stability at the Submolecular Level

Chiran Ghimire, Soyoung Park, Keisuke Iida, Philip Yangyuoru, Haruka Otomo, Zhongbo Yu, Kazuo Nagasawa, Hiroshi Sugiyama, and Hanbin Mao

The well-demonstrated biological functions of DNA G-quadruplex inside cells call for small molecules that can modulate these activities by interacting with G-quadruplexes. However, the paucity of the understanding of the G-quadruplex stability contributed from submolecular elements, such as loops and tetraguanine (G) planes (or G-quartets), has hindered the development of small-molecule binders. Assisted by click chemistry, herein, we attached pulling handles via two modified guanines in each of the three G-quartets in human telomeric G-quadruplex. Mechanical unfolding using these handles revealed that the loop interaction contributed more to the G-quadruplex stability than the stacking of G-quartets. This result was further confirmed by the binding of stacking ligands, such as telomestatin derivatives, which led to similar mechanical stability for all three G-quartets by significant reduction of loop interactions for the top and bottom G-quartets. The direct comparison of loop interaction and G-quartet stacking in G-quadruplex provides unprecedented insights for the design of more efficient G-quadruplex-interacting molecules. Compared to traditional experiments, in which mutations are employed to elucidate the roles of specific residues in a biological molecule, our submolecular dissection offers a complementary approach to evaluate individual domains inside a molecule with fewer disturbances to the native structure.