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Nanotube flow sensors generate a voltage
17 January 2003

Researchers from the Indian Institute of Science and India's Raman Research Institute have generated a voltage by flowing liquid down bundles of single-walled carbon nanotubes. The technique could be useful in flow sensors for small volumes of liquid, or as a voltage source in biomedical applications.

Nanotube researchers

"Our experiments have demonstrated for the first time that flow induces electrical response in single-walled carbon nanotubes," Ajay Sood of the Indian Institute of Science told nanotechweb.org. "The response of the nanotubes is relatively large even for very small flow velocities, of the order of 1 μm per second."

The researchers packed bundles of nanotubes with an average tube diameter of 1.5 nm in between two metal electrodes, producing a sensor 1 mm long, 0.2 mm thick and 2 mm wide. In order to prevent turbulent flow in the liquid, the scientists put the sensor at the centre of a glass tube roughly 30 mm in diameter and 0.9 m long.

Introducing water flowing at a speed of 5 μm/s induced a voltage of 0.65 mV in the device along the flow direction. As the liquid velocity increased, the voltage went up almost logarithmically - for example, water at speeds of 1.8 mm/s generated a voltage of 2.67 mV. The sensor had a response time of less than 1 ms.

The team found that liquids with a greater ionic strength induced higher voltages - for example 1.2 molar hydrochloric acid flowing at a speed of 0.1 mm/s produced five times more voltage than water at the same speed. Polar liquids such as water also developed much higher voltages than non-polar liquids such as methanol.

Multi-walled carbon nanotubes, meanwhile, generated a voltage about 10 times smaller than that produced by single-walled nanotubes under similar conditions.

The scientists believe that they will be able to scale down the sensor to micron-sized dimensions - the length of the individual nanotubes - enabling its use in very small liquid volumes, for example in labs-on-a-chip. "The unique point of this sensor is that it gives an electric response, which will be helpful in control electronics," added Sood. The devices could also be used as a voltage or current source in flowing liquids, possibly in biomedical applications.

About the author

Liz Kalaugher is editor of nanotechweb.org.