Browsing by Subject "Forces"
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Item Direct measurement of the energy landscape of ligand-receptor interactions(2010-08) Schwemmer, Frank Heinz, 1986-; Florin, Ernst-Ludwig; Shubeita, George T.In this thesis, a novel single molecule technique will be presented that will, for the first time, give direct access to the interaction energy landscapes of small molecules. The technique relies on the interpretation of thermal position fluctuations of a colloidal probe particle tethered to the molecular complex of interest and a geometrical amplification effect that converts Ångstrom scale fluctuations of the ligand in the binding pocket of the receptor to tens of nanometer fluctuation of the bead. The position of the bead is measured with 0.5 MHz bandwidth and 2 nm spatial resolution. The surface characteristic of the substrate was found to be critical for this new technique and various surface effects were observed. Methods were developed to block nonspecific interaction between the surfaces. The mobility of specifically bound particles was found to depend strongly on the density of specific bonds and the length of the molecular complex; low concentration and short linker lead to slow ligand-receptor mediated surface diffusion, high concentration and/or long linkers to an immobilization of the particle. Transient bond formation was observed for the intermediate range. Details of the interaction energy landscape were not resolved. However, a systematic change in the linker length from 22 Å to 29 Å led to a corresponding change in the lateral position fluctuations from 12.9 nm to 13.2 nm in excellent agreement with our theoretical calculations, confirming the geometrical amplification effect. Also, a new phenomenon of nanometer scale friction in the gap between the bead and the surface was discovered. In summary, the results underline that the novel technique might be able to measure details of the interaction energy landscape of a specific ligand-receptor bond and thus test theoretical predictions for its shape.Item Preliminary studies of the influence of forces and kinetics on interfacial colloidal assembly(Texas A&M University, 2004-11-15) Fernandes, GregoryIn this research we illustrate how particle-particle and particle-substrate interactions affect structure in interfacial colloidal systems. A number of tools are used to quantify characteristics of deposited structures. These results help understand the effects of colloidal system interactions and deposition kinetics on the degree of ordering in interfacial colloidal structures. The first set of experiments involve 2.34 ?m silica colloids interacting with silica substrates in 0mM, 5mM, 10mM, and 100mM NaCl solutions. Only the 100mM NaCl solution resulted in rapid deposition driven by van der Waals attraction, while residual electrostatic repulsion produced levitation at lower ionic strengths. This allowed direct observation of the effects of varying magnitudes of attractive interactions on interfacial colloidal structures. Rapid deposition of positively charged 1?m latex colloids on negatively charged silica substrates driven by Coulombic and van der Waals attraction produced surface structures similar to those obtained with only van der Waals attraction. Experiments on 2.34 ?m silica colloids interacting with silica substrates in 10mM NaCl/pH 5.5 and 10mM NaCl/pH 10 conditions resulted in slower deposition rates. It was also found that slower deposition rates produced more compact structures displaying a higher degree of order. Another set of experiments was aimed at understanding interactions and structures formed in systems of polymerically levitated particles. Total internal reflection microscopy (TIRM) experiments revealed the influence of underlying substrate chemistry on interaction profiles in these systems. Basic experiments were also performed on the effects of varying amounts of specific ions on the dispersion stability in these systems. At conditions producing instability in polymeric systems, a similar degree of order was observed in comparison to experiments involving rapid deposition via salt addition in electrostatically stabilized systems. The results of this research clearly indicate that particle-particle and particle-substrate interactions are critical in determining structure formation by deposition. While the principal focus of this research is to study structures formed in various kinetic regimes, it also provides a basis for future studies aimed at tuning attractive interactions to produce equilibrium colloidal crystals on substrates.Item Ultra-precise manipulation and assembly of nanoparticles using three fundamental optical forces(2012-12) Demergis, Vassili; Florin, Ernst-Ludwig; Shubeita, George T; Fink, Manfred; Makarov, Dmitrii E; Korgel, Brian AThe invention of the laser in 1960 opened the door for a myriad of studies on the interactions between light and matter. Eventually it was shown that highly focused laser beams could be used to con fine and manipulate matter in a controlled way, and these instruments were known as optical traps. However, challenges remain as there is a delicate balance between object size, precision of control, laser power, and temperature that must be satisfied. In Part I of this dissertation, I describe the development of two optical trapping instruments which substantially extend the allowed parameter ranges. Both instruments utilize a standing wave optical field to generate strong optical gradient forces while minimizing the optical scattering forces, thus dramatically improving trapping efficiency. One instrument uses a cylinder lens to extend the trapping region into a line focus, rather than a point focus, thereby confining objects to 1D motion. By translation of the cylinder lens, lateral scattering forces can be generated to transport objects along the 1D trapping volume, and these scattering forces can be controlled independently of the optical gradient forces. The second instrument uses a collimated beam to generate wide, planar trapping regions which can con fine nanoparticles to 2D motion. In Part II, I use these instruments to provide the first quantitative measurements of the optical binding interaction between nanoparticles. I show that the optical binding force can be over 20 times stronger than the optical gradient force generated in typical optical traps, and I map out the 2D optical binding energy landscape between a pair of gold nanoparticles. I show how this ultra-strong optical binding leads to the self-assembly of multiple nanoparticles into larger contactless clusters of well de ned geometry. I nally show that these clusters have a geometry dependent coupling to the external optical field.