A new modeling technique explains for the first time why a
single nanotube performs better than sensors containing several
nanotubes or flat planar sensors and refutes a popular explanation for
why smaller sensors work better than larger ones.
The technique was developed to study and design miniature
biosensors, and could help industry perfect lab-on-a-chip technology
for uses ranging from medical diagnostics to environmental monitoring.
The experimental devices represent a new class of portable
sensors designed to capture and detect specific "target molecules,"
which will allow the sensors to identify pathogens, DNA or other
Researchers at Purdue University are the first to
create "a new conceptual framework" and corresponding computational
model to relate the shape of a sensor to its performance and explain
why certain designs perform better than others, said Ashraf Alam, a
professor of electrical and computer engineering. Findings also refute
long-held assumptions about how to improve sensor performance.
The researchers tested and validated their model with
experimental data from various other laboratories.
"Many universities and companies are conducting experiments in
biosensors," Alam said. "The problem is that until now there has been
no way to consistently interpret the wealth of data available to the
research community. Our work provides a completely different
perspective on how to analyze their data and how to interpret them."
Research findings are detailed in a paper that appeared in the
Dec. 21 issue of the journal Physical Review Letters. The paper was
written by electrical and computer engineering doctoral student Pradeep
Nair and Alam.
Biosensors integrate electronic circuitry with natural molecules,
such as antibodies or DNA, which enable the devices to capture target
molecules. In efforts to design more sensitive devices, engineers have
created sensors with various geometries: some capture the biomolecules
on a flat, or planar surface, others use a single cylindrical nanotube
as a sensing element, and others use several nanotubes, arranged in a
crisscrossing pattern like overlapping sticks.
Researchers have known for several years that smaller devices
are more sensitive than larger ones. Specifically, the most sensitive
devices are those built on the scale of nanometers, or billionths of a
meter, such as tiny hollow nanotubes made of carbon.
"But we haven't really known why smaller sensors are more
sensitive," Alam said.
One obstacle in learning precisely why smaller sensors work
better is that the analysis is too computationally difficult to perform
with conventional approaches. The Purdue researchers solved this
problem by creating a model using a mathematical technique called
Cantor transformation, which simplified the computations needed for the
"That is the most important aspect of this work," Nair said.
"You could not effectively analyze the physics behind these biosensors
by using brute force with massive computing resources. It either could
not be done, or you would not be able to get consistent results."
The new model explains for the first time why a single
nanotube performs better than sensors containing several nanotubes or
flat planar sensors and refutes the predominant explanation for why
smaller sensors work better than larger ones.
"Everyone presumes that the nanometer-scale sensors are better
simply because they are closer to the size of the target molecules,"
Alam said. "This classical theory suggests that because larger sensors
dwarf the molecules they are trying to detect, these target molecules
are just harder to locate once they are captured by the probe. It's
like trying to see a small speck on a large surface. But that same
target molecule is no longer a speck if it lands on a probe closer to
its own size, so it's much easier to see.
"What we found, however, was not that smaller sensors are
better able to detect target molecules, but that they are better able
to capture target molecules. It's not what happens after the molecule
is captured that determines how well the sensor works. It's how fast
the sensor actually captures the molecule to begin with that matters
The distinction is important for the design of biosensors.
The reason smaller sensors capture molecules more effectively
is because using a single nanotube sensor eliminates a phenomenon
called "diffusion slow down." As a result, target molecules move faster
toward single nanotubes than other structures.
The new model determined that "the smaller the better," Alam
"This acceleration starts coming in when you make sensors on
the size scale of tens of nanometers. That is when you will get a real
Future work will concentrate on applying the model to the
performance of a "fractal sponge," which is a shape containing many
pores. Such a shape is important for applications in drug delivery and
The research has been supported by the National Science
Foundation (NSF) through the NSF-funded Network for Computational
Nanotechnology, based at the Birck Nanotechnology Center at Purdue's
Discovery Park. More recently, the work also has been funded by the
National Institutes of Health.
The researchers used computer resources on the nanoHub, an
Internet-based science gateway that provides access to advanced
simulation and software tools. The nanoHub is part of the Network for