Thesis (Ph.D.)--University of Rochester. School of Medicine & Dentistry. Dept. of Biochemistry and Biophysics, 2011.
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. Conventional polymeric membranes are 2-3
orders of magnitude thicker than pnc-Si membranes and lack well de ned pores. Because
of their nanoscale architecture, pnc-Si membranes display unique characteristics when
used as sieves for molecular separations and porous media for electroosmosis. In this
work, pnc-Si membranes have been investigated in these applications and compared to
conventional membrane materials.
Experiments show that pnc-Si membranes enable sharp and rapid separations of
molecules by di usion. The tight pore distributions allow for precise fractionation of
complex mixtures of small molecules and proteins. Di usion across the membrane proceeds
quickly, as the thickness of the membrane is on the same scale as the diameter of
the molecules. Molecule diameters that are 30% or less of the maximum pore diameter
encounter little hindrance and di use as if no membrane were present. Theoretical predictions
using pore distribution data corroborate these results. Molecules close in size to
the maximum pore size are hindered more than theory predicts, and this may be a result
of molecular adsorption that shrinks the e ective pore size. Additionally, theoretical
simulations indicate that the hindrance imparted by the pores in thicker conventional
membranes is magni ed by the long di usion times through the pores, thereby reducing
the resolution of separations and requiring longer times to reach equilibrium.
The innate negative charge of the pnc-Si membrane also plays a role in separations.
Experimental separations performed at low salt concentrations (<100 mM) signi cantly
di ered from those at higher salt concentrations. At lower salt concentrations, the
ionic shielding of the negative membrane surface potential is reduced and di usion is
influenced by charge interactions between the membrane and the species being tested.
Electrostatic e ects scale with the Debye length, a measure of the thickness of the
di use layer of counterions near the pore walls. Results show that negatively charged
double-stranded DNA is strongly hindered from di using through the pores at low
salt concentrations. The di usion of proteins at pH above their isoelectric point and
negatively charged nanoparticles is also reduced at low salt concentrations.
Ultrathin pnc-Si membranes are expected to present less resistance to fluid flow
as compared to micron thick conventional membranes. Experiments supported this
hypothesis, and pnc-Si membranes displayed water flow rates that were 2-3 orders of
magnitude higher than conventional thick membranes and at least 1 order of magnitude
higher than other nanoengineered membranes found in the literature. Theoretical predictions
of water flow were performed using pore distribution data. The theoretical and
experimental flow rates were found to be in good agreement. The tight pore distributions
enabled separation cuto s between proteins with a di erence of 2 nm in diameter
and gold nanoparticles with a di erence of 5 nm in diameter.
Finally, pnc-Si membranes were found to have normalized electroosmosis rates of
2.6 102 mL min๔1 cm๔2 V๔1, which are 2-3 orders of magnitude higher than other
DC electroosmotic pumps in the literature. The high
ow rates are attributed to the
fact that high electric elds form across the ultrathin membranes even with low applied
voltages. Agreement is shown between electroosmotic
ow rates and theory developed
for long pores, indicating that there is minimal contribution to the fluid flow from
entrance and exit e ects in the ultrathin membrane. Pnc-Si membranes add little
electrical resistance to the system and the transmembrane voltage drop is on the order
of 10 mV. With optimization, pnc-Si membranes could function as the rst low voltage,
on-chip electroosmotic pumps for microuidic devices.
