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.