Portable nitrogen oxide meter

Research at the University of Groningen has resulted in a prototype of a portable nitrogen oxide meter. This sensor can be used for medical applications, but may also be an interesting prospect for the car industry.

At present, complex and expensive equipment is needed to measure gaseous nitrogen oxides, which is part of the compulsory MOT test for cars. But University of Groningen PhD student Anne-Marije Andringa has come up with a smaller and simpler measuring instrument. Andringa developed a nitrogen oxide sensor based on a field-effect transistor. This type of transistor consists of a semiconductor, placed between two electrodes, that can conduct an electric current, a dielectric and a gate. Changing the voltage on the gate modulates the conductivity in the semiconductor.

Free electrons

Nitrogen dioxide penetrates the semi-conducting material and traps the free electrons needed to conduct electricity through the material, causing a drop in current. ‘The gate allows you to control the number of free electrons in the semiconductor’, explains Andringa. If you manipulate the number of free electrons in the semiconductor via the gate, you can maximize the sensitivity to nitrogen dioxide. Andringa devised a model that accurately describes the relationship between the concentration of nitrogen dioxide, the voltage on the gate and the current passing through the semiconductor.

‘The current generation of nitrogen dioxide sensors works with resistors. The resistance increases with the levels of nitrogen dioxide, but the increase is only a few percent’, says Andringa. In the field-effect transistor, nitrogen dioxide shifts the point at which the current switches ‘on’ and ‘off’, thereby producing a much stronger signal.

Practical applications

Although Andringa is not the first person to use a field-effect transistor to measure nitrogen dioxide, previous experiments did not aim to develop a calibrated sensor with practical applications. ‘I have devised a complete measuring protocol that is backed up by a model that translates the readout into the nitrogen dioxide concentration. I’ve verified the model experimentally and demonstrated a prototype sensor that can detect the concentration of nitrogen dioxide in real time.’

Portable sensor

The sensor is easy to integrate into modern micro-electronics, making all kinds of applications possible. ‘For example, you could build a nitrogen dioxide sensor into a car to check the effect of your catalytic converter.’ Another possible application involves asthma patients, who are known to show higher concentrations of nitric oxide in their exhaled breath a few days before a serious asthma attack. ‘At the moment, this can only be measured in a hospital. A portable sensor would enable patients to monitor their breath at home and adjust their medication if an attack is imminent.’

Andringa conducted her research at Philips Research in Eindhoven and was supervised by Prof. Dago de Leeuw, who combines a professorship in the Physics of Organic Semiconductors in the Zernike Institute for Advanced Materials of the University of Groningen with a research position at Philips Research. Andringa’s research was funded by the Netherlands Organisation for Scientific Research (NWO) and the University of Groningen. Laboratory facilities and support were provided by Philips Research.

Anne-Marije Andringa will be awarded a PhD by the Faculty of Mathematics and Natural Sciences of the University of Groningen on 8 February. Her thesis is entitled Gas sensing with field-effect transistors.

See also: ScienceLinX