"Nanotubes for differential-mass resonant sensors may also be able to detect extremely small changes in mass in a liquid because we can encase the liquid inside the tube," team member Nathaniel Quitoriano told nanotechweb.org.

Ideal mass sensors
Resonating CNTs could be ideal for making mass sensors thanks to their small radii. This is because the properties of the tubes significantly change when the devices deflect by a very small amount (for example, when a mass is placed on the tubes). The change in electronic properties of CNTs that occurs when such a strain is applied can then be detected and used for sensing applications.

Quitoriano and colleagues made their single-crystal silicon nanotubes by first growing a sacrificial germanium nanowire. Next, the researchers grew a thin silicon film, around 10 nm thick, on the germanium template. The tip of the nanowire was removed to expose the germanium core and the germanium selectively etched to leave behind the crystalline silicon nanotube.

Quality factor 1800
The as-produced nanotubes resonate well and are robust enough to survive multiple wet chemical processing steps. They have a quality factor, Q, of 1800, which is the ballpark useful for differential-mass sensors. The quality factor determines how sensitive the device is, because it is inversely proportional to the mass sensitivity.

The nanotubes could be used as chemical and biological differential-mass sensors thanks to their extremely small mass," said Quitoriano. "The sensitivity of such devices is roughly proportional to the mass of the sensor, so the smaller the mass of the sensor, the greater the sensitivity."

The researchers now plan to functionalize the nanotubes so that they can measure a particular chemical species.

The work, published in Nano Letters, is part of the Central Nervous System for the Earth (CeNSE) project at HP Labs.