PhD project on portable nitrogen oxide meter

Anne-Marije Andringa defended her PhD thesis at the University of Groningen earlier this month. She has designed and tested a portable nitrogen-oxide detector, which might one day be used by asthma patients and car manufacturers.

Andringa studied chemistry at the University of Groningen. ‘I was especially interested in polymer chemistry. I heard about plastic electronics during my Master’s degree and was fascinated by electronics made of organic molecules. You can use special plastic materials for all kinds of cool applications: flexible photovoltaic cells or OLEDs, for example.’

Andringa specialized in Physical Chemistry and Polymer Chemistry during her studies. Developing new materials to use in nanotechnology looked like real fun. ‘And the science behind it all is very interesting.’

She did a placement at Philips Research in Eindhoven to see how things worked there, and her placement supervisor, Dago de Leeuw, asked her to start a PhD project in Eindhoven. Dago de Leeuw combines his job as professor of Applied Physics at the University of Groningen with a Research Fellowship at Philips Research.

The project involved developing a new detector for gaseous nitrogen oxides. ‘The current systems are huge, complicated and expensive.’ It means that you have to have the nitric oxide levels in your car exhaust checked at the garage, because they have the right equipment. 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.

Nitrogen dioxide penetrates the semiconducting 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’, Andringa explains. If you manipulate the number of free electrons in the semiconductor via the gate you can maximize 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 work with resistors. The resistance increases with the levels of nitrogen dioxide, but only by 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.

As part of her project, Andringa built a demonstrator model of the nitrogen dioxide detector. Although other scientists have used field-effect transistors to detect nitrogen dioxide, Andringa is the first to produce a comprehensive calibrated system. ‘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.’

A portable system to measure nitrogen oxides could have several – very different – uses. ‘Car manufacturers could use it to monitor exhaust fumes during driving. But it has also been shown that nitric oxide levels increase in the exhaled breath of asthma patients days before a major attack.’ At present, you can only measure this in a hospital, so a portable system for home use would enable patients to check whether an asthma attack is imminent. ‘The beauty of the field-effect transistor is that it is fully compatible with standard electronic equipment like your mobile phone.’