Sensor, molecular device development focus of NSF funded research

December 20, 2001

BLACKSBURG, VA, DEC. 20, 2001 -- Two Virginia Tech research projects -- to develop new sensors for detecting pathogens and DNA, and to improve molecular devices in electronic applications -- received Nanoscale Exploratory Research (NER) grants from the National Science Foundation (NSF).

A nanometer (nm) is 10,000 times smaller than the width of a human hair. Research by Massimiliano Di Ventra of Virginia Tech's Department of Physics and a joint effort of Randy Heflin of physics and Kevin Van Cott of chemical engineering is exploring the nanoscale world through computer simulations and a combination of optics, thin-film technology, and analytical biochemistry.

Sensor technology

Heflin and Van Cott are attempting to develop new sensor approaches for detecting the presence of biological entities-such as pathogens, DNA, or biological compounds-in an environment or a sample.

The research using the NER grant will focus on the detection of DNA, Van Cott said. With the Human Genome Project in the spotlight and genomic-based medicine looking to see what genes are expressed in healthy versus diseased tissues, these findings will guide therapy and research dealing with those diseases. Most existing DNA screening systems rely on radioactive labeling or fluorescent labeling of the sample DNA, and this labeling brings in another experimental step that can possibly introduce uncertainties, he said.

"Our system does not require labeling of the sample DNA, and so we believe that our method, combined with the unique optical properties of the sensor platform, will be more sensitive, more reliable, and still have the high through-put, or ability to look for thousands of genes at the same time," Van Cott said

Heflin and Van Cott's research under the NER grant builds on a grant they currently have with Harry Gibson of chemistry and Rick Davis of chemical engineering at Virginia Tech. That project's goal is to develop self-assembled films of nanometer-scale thickness, particularly those films that have nonlinear optical properties, Heflin said. An example of such a property is second-harmonic (double) generation in which the researchers put light from a laser into a material and the material creates light at twice the frequency.

"In that project, the self-assembled films grow in alternate layers of two different materials," Heflin said. "One of the two materials is specific for nonlinear optical properties. The other material is pretty much a glue. We found that when we put on a layer of the second material, it causes a decrease in the signal we observe, in the intensity of light at the new harmonic frequency. Just the outermost layer causes the decrease. If we could actually find a way to specifically attach certain materials to the outer surface and have them cause a decrease, we would see the decrease if that target material was present, in a solution, for example. If the target material were not present, the signal would not decrease."

Basically, Heflin said, he and Van Cott are modifying the nonlinear optical (NLO) materials so that they have a biochemical group on them that recognizes only the target materials. "The sensor works on the idea that complementary biological molecules interact in very precise ways, analogous to a molecular-scale lock and key. We are investigating whether we can modify the NLO materials so that adsorption and binding of a complementary bio-molecule can be specifically detected," Heflin said.

"We propose to use complementary DNA molecules so that, if we want to look for a particular DNA sequence, we can take a complementary sequence, attach it to the NLO material, and deposit it in a nanometer-thick film. If the sample DNA is complementary, there will be a change in the intensity of light at the new frequency."

Molecular electronics

The problem with current silicon-based technology, says Di Ventra, is that, in the coming years, it will reach the physical limits of the number of transistors that can be integrated into a single chip. "The larger the number of transistors in a single chip, the larger our computational capacity," he said. "We need to come up with alternative solutions. Electron devices made up of molecular wires could solve the problem. Molecules are orders of magnitude smaller than current devices, and therefore we can put more of them into a single chip."

However, before researchers can actually employ molecular wires in electronics, they need to understand their electronic transport properties. That's where Di Ventra, a theoretical physicist, comes in. His goal is to understand how electrons behave when traveling into regions as small as a few atoms.

Di Ventra does computer simulations of the way electron devices work in transporting current. Before coming to Virginia Tech, he worked at Vanderbilt University and IBM T.J. Watson Research Center, where he developed theoretical tools to study electron transport in nanoscale structures.

"The proposal funded by the new NSF grant is related to one problem that is both fundamental and applied," Di Ventra said. When electric current passes through a device, the current can fluctuate around its average value. If these fluctuations, known as shot noise, are too large, the devices cannot be used in practice, he said. Di Ventra is attempting to understand the role of fluctuations in molecular devices.

"We must understand, if current fluctuations are large, how to make them small to develop more efficient molecular devices," he said. "That requires a novel theoretical approach." Since coming to Virginia Tech, Di Ventra has developed a theoretical scheme to study current fluctuations in nanoscale systems. The scheme allows a quantum-mechanical description of shot noise at the atomic level and therefore will shed more light on this phenomenon in molecular devices, he said. The grant will help with the computer implementation of this theoretical approach.

The NER grants were solicited within the National Nanotechnology Initiative last year, with only four grant submissions allowed per university. Di Ventra's, Heflin's and Van Cott's proposals were among the approximately 40 funded out of 260 submitted nationally. Each will receive $100,000 in seed money to spur exploratory nanoscience research. At the nanoscale level, scientists can possibly develop revolutionary ways of making materials and products that will greatly increase the speed of electrical processes and reduce the power needed to run the devices.

Heflin has been at Virginia Tech since 1992. He has received the Cottrell Scholar Award from Research Corporation and serves as one of the chairs of an annual conference on Organic Thin Films for Photonic Applications. He and Di Ventra are both associate editors of the International Journal of Nanoscience, a newly established journal devoted to the advancements in nanoscience and nanotechnology. The journal will be launched in 2002.
Reach Dr. Heflin at 540-231-4504 or,
Reach Dr. Di Ventra at 540-231-8729 or,
Reach Dr. Van Cott at 540-231-4257 or, or
PR Contact: Sally Harris,, 540-231-6759

Virginia Tech

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