updated: 5/14/2012 6:26:27 PM

Purdue Marks Scientific Advances

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Work by two teams of Purdue University researchers is being spotlighted in a national science publication. One group has developed new technology aimed at early cancer detection. The other is working on a product that could cost-effectively improve the performance of optical devices such as microscopes.

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May 14, 2012

News Release

West Lafayette, Ind. Researchers have created an ultrasensitive biosensor that could open up new opportunities for early detection of cancer and "personalized medicine" tailored to the specific biochemistry of individual patients.

The device, which could be several hundred times more sensitive than other biosensors, combines the attributes of two distinctly different types of sensors, said Muhammad A. Alam, a Purdue University professor of electrical and computer engineering.

"Individually, both of these types of biosensors have limited sensitivity, but when you combine the two you get something that is better than either," he said.

Findings are detailed in a paper appearing Monday (May 14) in the Proceedings of the National Academy of Sciences. The paper was written by Purdue graduate student Ankit Jain, Alam and Pradeep R. Nair, a former Purdue doctoral student who is now a faculty member at the Indian Institute of Technology, Bombay.

The device called a Flexure-FET biosensor - combines a mechanical sensor, which identifies a biomolecule based on its mass or size, with an electrical sensor that identifies molecules based on their electrical charge. The new sensor detects both charged and uncharged biomolecules, allowing a broader range of applications than either type of sensor alone.

The sensor has two potential applications: personalized medicine, in which an inventory of proteins and DNA is recorded for individual patients to make more precise diagnostics and treatment decisions; and the early detection of cancer and other diseases.

In early cancer diagnostics, the sensor makes possible the detection of small quantities of DNA fragments and proteins deformed by cancer long before the disease is visible through imaging or other methods, Alam said.

The sensor's mechanical part is a vibrating cantilever, a sliver of silicon that resembles a tiny diving board. Located under the cantilever is a transistor, which is the sensor's electrical part.

In other mechanical biosensors, a laser measures the vibrating frequency or deflection of the cantilever, which changes depending on what type of biomolecule lands on the cantilever. Instead of using a laser, the new sensor uses the transistor to measure the vibration or deflection.

The sensor maximizes sensitivity by putting both the cantilever and transistor in a "bias." The cantilever is biased using an electric field to pull it downward as though with an invisible string.

"This pre-bending increases the sensitivity significantly," Jain said.

The transistor is biased by applying a voltage, maximizing its performance as well.

"You can make the device sensitive to almost any molecule as long as you configure the sensor properly," Alam said.

A key innovation is the elimination of a component called a "reference electrode," which is required for conventional electrical biosensors but cannot be miniaturized, limiting practical applications.

"Eliminating the need for a reference electrode enables miniaturization and makes it feasible for low-cost, point-of-care applications in doctors' offices," Alam said.

A U.S. patent application has been filed for the concept.

The work has been funded by the U.S. Department of Defense, U.S. Department of Energy, National Institutes of Health-PRISM center at Purdue's Discovery Park, and the Semiconductor Research Consortium through the MSD center at the Massachusetts Institute of Technology.

Source: Purdue University

May 14, 2012

News Release

West Lafayette, Ind. - Researchers have taken a step toward overcoming a key obstacle in commercializing "hyperbolic metamaterials," structures that could bring optical advances including ultrapowerful microscopes, computers and solar cells.

The researchers have shown how to create the metamaterials without the traditional silver or gold previously required, said Alexandra Boltasseva, a Purdue University assistant professor of electrical and computer engineering.

Using the metals is impractical for industry because of high cost and incompatibility with semiconductor manufacturing processes. The metals also do not transmit light efficiently, causing much of it to be lost. The Purdue researchers replaced the metals with an "aluminum-doped zinc oxide," or AZO.

"This means we can have a completely new material platform for creating optical metamaterials, which offers important advantages," Boltasseva said.

Doctoral student Gururaj V. Naik provided major contributions to the research, working with a team to develop a new metamaterial consisting of 16 layers alternating between AZO and zinc oxide. Light passing from the zinc oxide to the AZO layers encounters an "extreme anisotropy," causing its dispersion to become "hyperbolic," which dramatically changes the light's behavior.

"The doped oxide brings not only enhanced performance but also is compatible with semiconductors," Boltasseva said.

Research findings are detailed in a paper appearing Monday (May 14) in the Proceedings of the National Academy of Sciences.

The list of possible applications for metamaterials includes a "planar hyperlens" that could make optical microscopes 10 times more powerful and able to see objects as small as DNA; advanced sensors; more efficient solar collectors; quantum computing; and cloaking devices.

The AZO also makes it possible to "tune" the optical properties of metamaterials, an advance that could hasten their commercialization, Boltasseva said.

"It's possible to adjust the optical properties in two ways," she said. "You can vary the concentration of aluminum in the AZO during its formulation. You can also alter the optical properties in AZO by applying an electrical field to the fabricated metamaterial."

This switching ability might usher in a new class of metamaterials that could be turned hyperbolic and non-hyperbolic at the flip of a switch.
"This could actually lead to a whole new family of devices that can be tuned or switched," Boltasseva said. "AZO can go from dielectric to metallic. So at one specific wavelength, at one applied voltage, it can be metal and at another voltage it can be dielectric. This would lead to tremendous changes in functionality."

The researchers "doped" zinc oxide with aluminum, meaning the zinc oxide is impregnated with aluminum atoms to alter the material's optical properties. Doping the zinc oxide causes it to behave like a metal at certain wavelengths and like a dielectric at other wavelengths.
The material has been shown to work in the near-infrared range of the spectrum, which is essential for optical communications, and could allow researchers to harness "optical black holes" to create a new generation of light-harvesting devices for solar energy applications.

The PNAS paper was authored by Naik, Boltasseva, doctoral student Jingjing Liu, senior research scientist Alexander V. Kildishev, and Vladimir M. Shalaev, scientific director of nanophotonics at Purdue's Birck Nanotechnology Center, a distinguished professor of electrical and computer engineering and a scientific adviser for the Russian Quantum Center.

Current optical technologies are limited because, for the efficient control of light, components cannot be smaller than the size of the wavelengths of light. Metamaterials are able to guide and control light on all scales, including the scale of nanometers, or billionths of a meter.
Unlike natural materials, metamaterials are able to reduce the "index of refraction" to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. It causes the bent-stick-in-water effect, which occurs when a stick placed in a glass of water appears bent when viewed from the outside. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material

Natural materials typically have refractive indices greater than one. Metamaterials, however, can make the index of refraction vary from zero to one, which possibly will enable applications including the hyperlens.

The layered metamaterial is a so-called plasmonic structure because it conducts clouds of electrons called "plasmons."

"Alternative plasmonic materials such as AZO overcome the bottleneck created by conventional metals in the design of optical metamaterials and enable more efficient devices," Boltasseva said. "We anticipate that the development of these new plasmonic materials and nanostructured material composites will lead to tremendous progress in the technology of optical metamaterials, enabling the full-scale development of this technology and uncovering many new physical phenomena."

This work has been funded in part by the U.S. Office of Naval Research, National Science Foundation and Air Force Office of Scientific Research

Source: Purdue University

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