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New composite forms basis of simple carbon dioxide sensor

08 June 2015

Material scientists at ETH Zurich and the Max Planck Institute of Colloids and Interfaces have developed an entirely new type of sensor that can measure CO2.

ETH researchers' miniature CO2 sensor is a chip with a thin layer of the polymer-nanoparticle composite (photo: Fabio Bergamin/ETH Zurich)

The new sensor consists of a recently developed composite material that interacts with CO2 olecules and changes its conductivity depending on the concentration of CO2 in the environment. The scientists have created a sensor chip with this material that enables them to determine CO2 concentration with a simple measurement of electrical resistance.

The basis of the composite material is a chain-like macromolecule (polymer) made up of salts (ionic liquids), which are liquid and conductive at room temperature. The name of the polymers is slightly misleading as they are called 'poly(ionic liquid)s' (PIL), although they are solid rather than liquid.

The researchers - Christoph Willa and Dorota Koziej - mixed the polymers with specific inorganic nanoparticles that also interact with CO2. By experimenting with these materials, they were able to produce the composite. “Separately, neither the polymer nor the nanoparticles conduct electricity,” says Willa. “But when we combined them in a certain ratio, their conductivity increased rapidly.”

They also discovered that the conductivity of the composite material at room temperature is CO2-dependent. “Until now, chemoresistive materials have displayed these properties only at a temperature of several hundred degrees Celsius,” says Koziej.

Exactly how the CO2-dependent changes in conductivity were produced is not yet clear; however, the scientists have found indications that a chemical change induced by the presence of CO2 occurs foremost at the interface between the nanoparticles and the polymers at the nanometre  scale. “We think that CO2 affects the mobility of the charged particles in the material,” says Koziej.

With their new sensor, the scientists are able to measure CO2 concentration over a wide range – from a concentration of 0.04 volume percent in the earth’s atmosphere to 0.25 volume percent.

Existing devices that can detect CO2 are optically based and capitalise on the fact that CO2 absorbs infra red light. In comparison, researchers believe that with the new material much smaller, portable devices can be developed that will require less energy.

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