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Ion and Particle Transport in Nanopores

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Nanopores exhibit behaviors not seen in the micro-scale, such as ion current rectification or ion selectivity, which are useful for biotechnology, building of ionic circuits, and other fields. Nanopores in polymer films prepared by the track etched technique are useful due to the ease which one can scale from a single nanopore to 109 pores/cm2. In addition, it is straightforward to control pore diameters, geometry, and pore wall chemistry. In this document, polymer membranes were used for two applications. For the first application, the undulating pore walls of single pores in a polyethylene terephthalate film were exploited to extend the resistive-pulse technique to distinguish between spherical and aspherical particles. In the resistive-pulse technique, particles are driven through the pore by an electrical potential while the ion current is continuously monitored. The particle's presence in the pore is detected as a current decrease, called a resistive-pulse. Particles which pass through pores with undulating diameters has a pulse whose shape reflects the pore topography. The pulses of spherical and aspherical particles are compared in both rough and smooth pores, and it was determined that undulating diameters are crucial to distinguishing between the two types of particles. In the second application, polymer membranes with porosity of 108 pores/cm2 are used for desalination in reverse osmosis experiments. Although the narrow opening of the conical pores are larger than the classical Debye length, it was found that with high surface charge densities, ion rejection still occurs. Ion rejection was found to further increase if the narrow opening was modified to positive charges, forming a bipolar junction on the pore wall. Lastly, single pores with diameters between 10-15 nm were drilled into 30 nm thick silicon nitride membranes to study low aspect ratio systems. While a bare silicon nitride pore exhibited no ion rectification behavior, when a gold layer is deposited at one pore opening, rectification was observed. This is due to the polarization of the gold, which generated regions of enhanced concentrations of ions in the pore, breaking the pore's electrochemical symmetry. The research demonstrates interdisciplinary character and a wide range of applications of nanopores.

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