BME PhD Defense: Jessica Snyder

Porous Nanocrystalline Silicon Membranes as Sieves and Pumps

Supervised by Professor Jim McGrath

Abstract

Porous nanocrystalline silicon (pnc-Si) is a novel nanoporous material that is fabricated into 7-30 nm thick freestanding membranes. Pores self assemble during the fabrication process, and pore distributions can be tuned between 5 and 100 nm, which is within the size range of small molecules and proteins. Because of their nanoscale architecture, pnc-Si membranes display unique characteristics when used as sieves for molecular separations and porous media for electroosmosis. Experiments show that pnc-Si membranes enable sharp and rapid separations of molecules by diffusion. The ability to view the pore distributions using transmission electron microscopy allows for comparison of diffusion separations with hindrance theory. The innate negative charge of the pnc-Si membrane also plays a role in separations. Experimental separations performed at low salt concentrations (<100 mM) significantly differed from those at higher salt concentrations. Pressurization of pnc-Si membranes produced flow rates 2-3 orders of magnitude higher than thicker nanoporous membranes, and sharp separations of gold nanoparticles and proteins were obtained. Finally, pnc-Si membranes were found to have electroosmotic flow rates that are 2-3 orders of magnitude higher than other DC electroosmotic pumps in the literature. The high flow rates are attributed to the fact that high electric fields form across the ultrathin membranes even with low applied voltages. With optimization, pnc-Si membranes could function as the first low voltage, on-chip electroosmotic pumps for microfluidic devices.