SENSOR technology keeps advancing with the development of smaller and smaller sensors that have higher and higher sensitivities. However, when it comes to conventional chemical detectors, most (if not all) still require a power source, that is, a battery, and therein lies a challenge. Battery-powered sensors require regular maintenance and replacement, making them problematic for field use. And, with sensors shrinking in size, the power source is often larger than the sensor itself, which defeats the purpose of miniaturization.
Fortunately, Livermore materials scientist Morris Wang and colleagues have found a way to bypass the power source requirement. Their “batteryless” nanosensor can identify different chemical species in less than a second, giving it potential for homeland security and medical applications.
Scientific Serendipity at Work
On seeing the strong signal, Livermore’s Wang thought something odd might be going on. He explains, “I always tell my postdocs that getting unusual, unexpected, or abnormal experimental results can often be a very good thing. It may mean they have discovered something new. The weirder the results, the more exciting they may be. In this particular case, we had a very weird result.” Upon further examination, they discovered that the nanowires—which poke out of the polymer base like teeny fingers—were interacting directly with the alcohol molecule.
From Alcohol to TNT
In the first platform, the 6- to 7-micrometer-long ZnO nanowires were infiltrated with a polyvinyl chloride polymer and etched with oxygen plasma, leaving exposed “fingers” about 0.1 to 0.5 micrometers tall. For the sensing experiments, a gold–titanium film and silver paste were used to make the top and bottom electrical contacts. The scientists then monitored the change in electric potential on the two ends of the nanowire.
The second platform used randomly aligned Si nanowires up to tens of micrometers long. About 80 percent of the nanowire tangle was sealed with polymer, leaving the remaining 20 percent exposed. Gold–titanium film was evaporated on two opposite sides of the substrate as electrical contacts. In this system, scientists monitored the change in electrical potential between the exposed and unexposed nanowires.
The team experimented with more than 15 different organic solvents, including acetone, chloroform, toluene, and ethanol, by dripping each chemical onto a sensor at room temperature. With ethanol, for example, the electric voltage rose sharply, peaking at approximately 170 millivolts. The signal rise was almost instantaneous and decayed slowly to zero as the ethanol evaporated. Other solvents produced different characteristic signals, yielding voltage “fingerprints” for each substance.
Wang then worked with engineer Chris Spadaccini in the Center for Micro- and Nanotechnology to test the nanowire device on explosives such as TNT (trinitrotoluene) and RDX (1,3,5-trinitro-1,3,5-triazacyclohexane). The results were very encouraging. The sensor was able to distinguish between different types of chemical explosives, which offers many new possibilities. “The device is potentially very sensitive and fast,” says Spadaccini. “In addition, because it is so small, it could be easily integrated into a handheld system.” Alex Hamza, director of the Nanoscale Synthesis and Characterization Laboratory, also sees the future benefits of batteryless technology, adding, “Developing techniques to power sensors and other nanostructured devices is vitally important to furthering their widespread use.”
How It Happens
To better understand what was happening at the atomic level, Wang turned to physicists Daniel Aberg and Paul Erhart in the Physical and Life Sciences Directorate. Aberg and Erhart focused on the chemisorption effects of the ethanol molecule to look for an explanation. In chemisorption, a chemical coats an exposed surface of a material, and the chemical and surface molecules create new electronic bonds. This process results in a new chemical species that forms a thin surface film, sometimes only one molecule thick. Corrosion and oxidation are two common products of the chemisorption process.
Aberg and Erhart used HERA, a large-capacity Linux cluster in Livermore’s Open Computing Facility, to run quantum mechanical calculations on detailed interactions at the atomic level. Results from the calculations indicated that the ethanol molecule had a negative adsorption energy when interacting with the top or side surfaces of the ZnO molecule. Aberg and Erhart experimented with a dozen or so different positions before determining the lowest energy level for the system. In this state, the oxygen atom in ethanol bonds with a zinc atom in the nanowire. Another bond (a hydrogen-bridge bond) forms between a hydrogen atom in ethanol and an oxygen atom in the nanowire.
The researchers also found that the degree of detection sensitivity is linked to the polarity of the molecules involved. “Signal strength is directly tied to the dipole moment of the molecule,” says Wang. Many molecules have a dipole moment, which is a function of the molecule’s asymmetric electrical nature. Water, for example, has a negatively charged oxygen atom at one end and two positively charged hydrogen atoms at the other. As a result, water molecules have a different charge at each end, and thus a polarity, much like the negative and positive poles of a battery. When molecules with a given dipole moment attach to a surface created of molecules that also have a dipole moment, the attaching molecules bind in a preferred direction. “The strength of the signal is determined by the dipole moment of the attaching chemical species and the surface area of the nanowire,” says Wang.
Charging Forward with Batteryless Nanowires
Batteryless, small, fast, inexpensive, and simple, these nanosensors are on their way to “electrifying” the chemical sensor field. The sensors made the inside cover of Advanced Materials last year, and commercialization of the technology is a possibility. Born out of researchers’ willingness to explore an out-of-place result from a different experiment, the batteryless chemical nanosensor is proof that it pays to heed scientific serendipity and see where the unexpected may lead.
Key Words: battery, batteryless, Center for Micro- and Nanotechnology, chemical detector, chemical sensor, homeland security, medicine, nanoscience, nanowire, power source, semiconductor.
For further information contact Yinmin (Morris) Wang (925) 422-6083 (firstname.lastname@example.org).
Lawrence Livermore National Laboratory
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