Fine balance

A pH sensor that uses carbon nanotubes instead of glass may revolutionise the pharmaceutical and food and drink industries. Siobhan Wagner reports


A sensor originally developed for monitoring pH in oil wells is being adapted for the pharmaceutical, food and beverage and chemical manufacturing industries.

The technology, which was invented by Richard Compton at Oxford University’s Physical and Theoretical Chemistry Laboratory, is being further developed and commercialised by San Francisco company Phathom Nanosensors.

Oxford’s technology transfer company Isis Innovation has licensed the technology to Phathom.

The technique replaces traditional glass electrode pH sensors with one that uses an electrode made of modified carbon nanotubes on a graphite surface.

‘Our technology has several key advantages over the glass electrode technology still widely used in industry,’ said Compton.

He claimed his sensor is more accurate, which enables tighter control of pH-critical manufacturing processes.

Unlike glass electrodes, the sensors are also self-calibrating.

Jamie Ferguson, project manager at Isis, explained that the sensor works using ‘working, reference and counter’ electrodes. The working electrode is made of modified carbon nanotubes deposited on a graphite surface. ‘The advantage of nanotubes is they give you a huge surface area,’ he said.

The reference electrode, an organic compound called anthraquinone, responds to the changes in the pH level. The counter electrode is polyvinylferrocene, a material that does not change its redox chemistry when the pH changes.

When the sensor is put into a liquid solution a voltage is created and the current response from the pH sensitive and pH insensitive material will differ. ‘The system monitors the difference between the current response to each of those materials, does a quick calculation and gets the pH from that,’ said Ferguson. ‘The clever thing is it measures the difference between a material which responds to pH and one that doesn’t, which is why you get the self-calibration.’

This feature, say the developers, gives these sensors an advantage over glass pH electrodes in, for example, the pharmaceutical industry.

‘When you’re making pharma-ceuticals by biotech methods you are typically brewing things up for several weeks — using bugs to make drugs,’ said Ferguson.

‘Over several weeks the conventional technology can drift out of calibration. You could also break a glass electrode and have a problem with glass in something that you are either going to feed people or give to people.’

He added: ‘With this technology, you have the advantage of not having to calibrate things and not worrying about whether they will break. In the food and drink industry that is a big advantage.’

Lee Leonard, chief executive of Phathom, said his company is excited about commercialising the sensor technology because it has the potential to shake up a market that is worth nearly $1bn (£555m) annually, yet still dominated by a 70-year-old technology.

‘Introduction of these sensors will be analogous to the replacement of vacuum tubes with transistors,’ he said. ‘With that comes the opportunity not only to improve existing processes already using pH measurement and control, but also to address new opportunities where the existing technology cannot be used due to calibration, drift and mechanical limitations.’

Compton’s research group has experience of developing electrochemical sensors for precise detection. Their previous invention measured the amount of ‘drugs of impairment’ such as cannabis and amphetamines by rapidly analysing a small sample of saliva.

Ferguson said the system would be comparable to breathalysers and could be used by the police to test drivers who appear to be under the influence of drugs. That technology is being further developed by Oxford spin-out, Oxtox.