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 atoms.  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 diagnosis, 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.

Fig 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.