Figure 1. Schematic of nanoelectromechanical (NEMS) gas sensors. A NEMS resonator is exposed to gas molecules (red and green spheres). When the molecules stick to the NEMS, its resonant vibration frequency drops (see lower inset). By using special chemical coatings on the NEMS, it can be made so that only certain chemicals will stick to the sensor; a measured mass change is thus a signal that a particular chemical is in the environment. The upper inset shows a scanning electron micrograph of a NEMS "diving board" style resonator; it is approximately 600 nanometers in length, 10 times smaller than a human red blood cell. This NEMS vibrates at a resonant frequency of approximately 120 million cycles per second.

The technology in this project uses tiny mechanical resonators, which are physical structures that vibrate at a particular frequency. Everyday examples of mechanical resonators include the strings of a violin and the rim of a wine glass, which emits a distinctive tone when rubbed. Our resonators, however, are more than a billion times smaller, smaller even than a human red blood cell (see Figure 1). Because they are so small, they vibrate much faster than everyday resonators; instead of audio frequencies, they vibrate at the frequencies of TV or radio waves, up to 200 million times per second.

Figure 2. Schematic of an "electronic nose."A particular gas chemical interacts with a large array of NEMS sensors, each with a different chemical coating. From each sensor comes a distinct response signal. By aggregating the set of signals from the NEMS array, a distinct chemical "fingerprint" can be constructed for any particular chemical. This method of chemical identification is similar to the way the human body performs olfaction, with our nanofabricated NEMS sensors taking the roles of the natural chemical receptors found in the human nose.

If we add mass to a resonator, its vibration frequency will be reduced. For everyday resonators, it may require grams or even kilograms of added mass to significantly reduce the vibration frequency. Our resonators, however, are so tiny that we can see a change in frequency when we add masses as small as a few thousand atoms¹. We can chemically tailor the surfaces of our resonators so that only particular kinds of atoms or molecules will stick to them. In this way we can use these devices as extremely sensitive gas sensors, to detect when particular chemicals are in the surrounding air. By combining large numbers of resonators with specialized coatings, a broad variety of gases can be detected, leading to the creation of what is called the "electronic nose", an artificial version of our own olfactory system (Figure 2). Such technology has significant and broad application potential, from detecting chemical, biological, and explosive weapons in military applications, to measuring environmental toxins. Electronic noses can even be used for biomedical use in breath-based disease diagnosis², where the presence of particular chemicals in human breath can be measured to do early-stage detection of diseases such as lung and breast cancer.

Personnel
Edward Myers, Ph.D. (team leader)
Xinchang Zhang, Ph.D. (GC/electronics specialist)
Derrick Chi (nanofabrication engineer)
Heather C. McCaig (Chemistry grad student, polymer functionalization)
Caryn Bullard (Physics grad student, nanostructured surface physics)
Haekong Kim (Physics grad student, electrochemical functionalization)

This work has primarily been supported by DARPA/MTO-MGA through grant NBCH1050001.

References

1. "Ultra-sensitive NEMS-based cantilevers for sensing, scanned probe and very high-frequency applications," Mo Li, H. X. Tang, and M. L. Roukes, Nature Nanotechnology 2, 114-120 (28 Jan 2007).
2. "Breath tests in medicine," Michael Phillips, Scientific American, July 1992.