Portable nitric oxide meter

Research at the University of Groningen has resulted in a prototype of a portable nitric 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 nitric 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 nitric oxide sensor based on a field effect transistor. This type of transistor consists of a semiconductor that can convey an electric current, a dielectric and a gate. Changing the voltage on the gate enables the conductivity in the semiconductor to be adjusted.

Free electrons

Nitric dioxide penetrates the semi-conducting material and traps the free electrons needed to conduct electricity through the material, causing a drop in conductivity. ‘The gate allows you to make an accurate reading of the number of free electrons in the semiconductor’, explains Andringa. Manipulating the number of free electrons in the semiconductor via the gate makes it possible to maximize sensitivity to nitrogen dioxide. Andringa devised a model that accurately monitors 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 resistance, which increases in line with the levels of nitrogen dioxide. But the increase in resistance is only a few percent’, says Andringa. In the field effect transistor, nitrogen dioxide switches the current ‘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 backed up by a model that monitors precisely what is happening and shows the concentration linked to a particular readout. 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 nitrogen monoxide in their 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 Science and Engineering (formerly known as 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.