Graduate student Judson Ryckman demonstrating how one of the biosensors works that was made by the direct imprinting of porous substrates process. Photo: Anne Raynor/Vanderbilt Univ. |
A simple technique for stamping patterns invisible to the
human eye onto a special class of nanomaterials provides a new, cost-effective
way to produce novel devices in areas ranging from drug delivery to solar
cells.
The technique was developed by Vanderbilt Univ.
engineers and described in Nano Letters.
The new method works with materials that are riddled with
tiny voids that give them unique optical, electrical, chemical, and mechanical
properties.
For a number of years, scientists have been investigating
the use of porous nanomaterials for a wide range of applications including drug
delivery, chemical and biological sensors, solar cells, and battery electrodes.
There are nanoporous forms of gold, silicon, alumina, and titanium oxide, among
others.
Simple stamping
A major obstacle to using the materials has been the complexity and expense of
the processing required to make them into devices.
Now, Associate Professor of Electrical Engineering Sharon M.
Weiss and her colleagues have developed a rapid, low-cost imprinting process
that can stamp out a variety of nanodevices from these intriguing materials.
“It’s amazing how easy it is. We made our first imprint
using a regular tabletop vise,” Weiss said. “And the resolution is surprisingly
good.”
The traditional strategies used for making devices out of
nanoporous materials are based on the process used to make computer chips. This
must be done in a special clean room and involves painting the surface with a
special material called a resist, exposing it to ultraviolet light or scanning
the surface with an electron beam to create the desired pattern and then
applying a series of chemical treatments to either engrave the surface or lay
down new material. The more complicated the pattern, the longer it takes to
make.
Electrical engineering professor Sharon Weiss. Photo: Neil Brake/Vanderbilt Univ. |
About two years ago, Weiss got the idea of creating
pre-mastered stamps using the complex process and then using the stamps to
create the devices. Weiss calls the new approach direct imprinting of porous
substrates (DIPS). DIPS can create a device in less than a minute, regardless
of its complexity. So far, her group reports that it has used master stamps
more than 20 times without any signs of deterioration.
Process can produce
nanoscale patterns
The smallest pattern that Weiss and her colleagues have made to date has
features of only a few tens of nanometers. They have also succeeded in
imprinting the smallest pattern yet reported in nanoporous gold, one with 70-nm
features.
The first device the group made is a “diffraction-based”
biosensor that can be configured to identify a variety of different organic
molecules, including DNA, proteins, and viruses. The device consists of a
grating made from porous silicon treated so that a target molecule will stick
to it. The sensor is exposed to a liquid that may contain the target molecule
and then is rinsed off. If the target was present, then some of the molecules
stick in the grating and alter the pattern of reflected light produced when the
grating is illuminated with a laser.
According to the researchers’ analysis, when such a
biosensor is made from nanoporous silicon it is more sensitive than those made
from ordinary silicon.
The Weiss group collaborated with colleagues in chemical and
biomolecular engineering to use the new technique to make nano-patterned
chemical sensors that are ten times more sensitive than another type of
commercial chemical sensor called Klarite that is the basis of a multimillion-
dollar market.
The researchers have also demonstrated that they can use the
stamps to make precisely shaped microparticles by a process called “over-stamping” that essentially cuts through the nanoporous layer to free the
particles from the substrate. One possible application for microparticles made
this way from nanoporous silicon are as anodes in lithium-ion batteries, which
could significantly increase their capacity without adding a lot of weight.
Vanderbilt
Univ. has applied for a
patent on the DIPS method.
