Doctoral Dissertations

Date of Award

8-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Lloyd M. Davis

Committee Members

Christian G. Parigger, William H. Hofmeister, Robert N. Compton

Abstract

This dissertation presents the development of an instrument for effectively trapping a single fluorescent nanoparticle that is freely diffusing in solution in all three dimensions. The instrument is expected to have applications for studies of single nanoparticles or molecules for which prolonged observations are required, but without immobilization or proximity to a surface, which may alter behavior. The trapping technique depends on rapid three-dimensional position measurements of the nanoparticle with sub-micron precision, which are used for real-time control of induced electrokinetic motion, so as to counteract Brownian motion. While anti-Brownian electrokinetic trapping experiments in one and two dimensions have previously been reported, this is the first account of three-dimensional electrokinetic trapping. A key innovation is the use of a custom microfluidic device with four electrodes in a tetrahedral arrangement spaced by about 100 microns. Adjustment of voltages between the four electrodes induces electrokinetic motion of the nanoparticle controllable in all three dimensions. To accomplish trapping, the device is mounted on a custom fluorescence microscope, in which the tube lens is tilted to deliberately introduce astigmatism. The tilt produces an elliptical point spread function when the nanoparticle is displaced from the focal plane. With use of calibration measurements, the position and shape of the point spread function from a camera image give the three-dimensional displacement of the nanoparticle. The electrode potentials to generate a proportional restoring motion are then applied. A 20-nanometer radius particle in aqueous solution can thus be held for a mean time of 7 seconds, which is much longer than the diffusional escape time without control. Statistical results over many such experiments show (x, y, z) fluctuations of (2.2, 1.8, 3.0) microns standard deviation from the target position, which corresponds to effective spring constants of (0.8, 1.2, 0.4) nanoNewtons per meter. In addition to trapping, arbitrary three-dimensional manipulation of the nanoparticle trajectory is demonstrated. Simulations show that time delay between measuring displacement and applying corrective motion requires reduced response to avoid instability and that use of the device with a faster camera or other position determination method should enable trapping of a 1.5 nanometer-sized objects in water.

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