Thursday, June 05, 2008

A Tiny Sensor Simply Made

A nanoscale biosensor that can detect minute amounts of pathogens could come to market this year.

Researchers at NASA Ames Research Center have developed a nanotechnology-based biosensor that can detect trace amounts of as many as 25 different microorganisms simultaneously and within minutes. The researchers make the biosensors by growing carbon nanofibers--a material with the same properties as carbon nanotubes but with a slightly larger diameter--using a process similar to the one employed to fabricate computer chips.

"By using the same reactor technology the semiconductor industry uses, we have created an innovative approach to manufacturing tiny sensors," say Meyya Meyyappan, the chief scientist for the biosensor project.

While NASA plans to eventually use the sensor to detect the presence of life on other planets, it has licensed the technology to Early Warning, a company based in Troy, NY, that develops systems to monitor biohazards. The company's president, Neil Gordon, says that the first application for the sensor will be for water-quality monitoring, and a prototype of the technology will be tested at a series of demonstration sites this summer. Early Warning plans to have a commercial product by the end of the year.

Detecting biohazards: A prototype of NASA’s nanotechnology-based biosensor has been licensed to Early Warning. The company is incorporating the technology into a device that will test for common and rare strains of microorganisms associated with waterborne illnesses in municipal water systems.
Credit: NASA

It has been known for almost a decade that carbon nanotubes and nanowires make good sensors, says Mark Reed, a professor of electrical engineering and applied physics at Yale University. But, says Reed, only in the past couple of years have research groups started to explore an integrated approach--electronics and biology--to build biosensing devices. Reed's group is among those using an integrated approach to build nanoscale sensors based on carbon nanowires. Harvard researchers led by chemist Charles Lieber were the first to show virus detection and the detection of early signs of cancer through semiconducting nanowires. Other academic groups, such as those at California Institute of Technology, the University of Southern California, and Boston College, are doing similar work.

Such biosensors, which are based on the detection of electrical signals, offer several advantages over more conventional optical technologies. For one thing, the electronic and electrochemical approaches do not require the use of florescent chemical tags, says Charlie Johnson, a professor in the department of physics and astronomy at the University of Pennsylvania. Electrical signals are also easier to measure than optical ones, he says.

NASA, however, is one of the few groups using carbon nanofibers to make biosensors. Carbon nanofibers are easier to work with than nanotubes are, and they can be grown on a silicon substrate in the exact structure that researchers desire, says Meyyappan.

Indeed, the real challenge to making electronic-based biosensors into products is not which material performs the best, but how they will be mass-produced, says Reed. "I am impressed by NASA's work, and they have very nice results," he says.

To make their sensors, NASA researchers start by coating a silicon wafer with a metal film like titanium or chromium. Next, the researchers deposit a catalyst of iron and nickel on top of the metal film, patterning the catalyst using conventional lithography. This allows the researchers to determine the location of the nanofibers, which will act as nanoelectrodes. A chemical vapor deposition process is used to grow the nanofibers on the catalyst.

"The proper construction and orientation of the nanoelectrode is critical for its electrochemical properties," says Meyyappan. "We want to grow the nanofibers in an array like telephone poles on the side of a highway--nicely aligned and vertical."

The researchers then place silicon dioxide in between the nanofibers so that they do not flap when they come in contact with fluids, like water and blood; this also isolates each nanoelectrode so that there is no cross talk. Excess silicon dioxide and part of the nanofibers are removed using a chemical mechanical polishing process so that only the tips of the carbon nanofibers are sticking out. The researchers can then attach a probe or molecule designed to bind the targeted biomolecule to the end of the nanofiber. The binding of the target to the probe generates an electrical signal.

The sensor is also equipped with conventional microfluidic technology--a series of pipes and valves--that will channel small drops of water over to specific probes on the biosensor side. This allows the researchers to do field testing and avoid the expense of taking the biosensor to the lab, says Meyyappan.

After the sensor is tested in its facilities this summer, Early Warning plans to place the device within an already existing wireless network to monitor the water quality of municipal systems. "The sensor gives us the advantage of having a lab-on-a-chip technology that can test for many different microorganisms in parallel," says Gordon. "And instead of waiting 48 hours for results, we get notified within 30 minutes if the water is contaminated," he says.

Such sensors could also be used in homeland security to detect pathogens such as anthrax, to detect viruses in air or food, and for medical diagnostics, says Meyyappan.