Electricity and some gases and liquids can be a dangerous - even explosive - combination. But sensors are still needed in harsh gaseous and liquid environments, whether to check for leaking hydrogen fuel cells or to measure the composition or acidity of industrial chemicals. EU-funded researchers have developed an optical solution that is safer, easier to install and, unexpectedly, much more sensitive than most existing chemical sensors.
Developed in the Dotsense * project, the sensors are based on an innovative application of quantum dots and nanowires - miniscule semiconductors whose features are thousands of times smaller than the width of a human hair. Made of the group III-nitride semiconductor system - (Al,In)GaN - chemically stable semiconductor materials with excellent opto-electronic properties, these structures show changes in their photoluminescence properties when exposed to even the smallest changes in the chemical environment.
'To date, many approaches have been adopted for sensing technology, including, for example, using nanowires as chemical sensors, but these approaches are based on measuring electrical conductivity. This means you have to put in electrical contacts and measure the change in the electrical resistance of the nanowire in different chemical environments,' explains Dr. Martin Eickhoff, the Dotsense project coordinator at Justus-Liebig-University in Giessen, Germany. 'With our approach, that's unnecessary. Our solution is based on a completely optical analysis.'
Instead of running an electrical current through the nanostructures and measuring the resistance, the Dotsense team created an integrated sensor system that works solely with light.
An optical transducer, made of an array of a billion GaN or InGaN 'quantum dots' or 'nanodisks' in nanowires, is placed inside the gaseous or liquid environment that is to be monitored and an excitation light is shone through a transparent substrate that simultaneously serves as a sealing window. The photoluminescence properties of the nanostructures change depending on which chemicals are present in the environment being monitored, hence varying the intensity of the light emitted from the transducer. The change can then be read out using commercially available photodetectors.
'We take advantage of the chemical sensitivity and the high surface-to-volume ratio of the nanostructures without having to implement a more complicated processing technology - there's a lot less technological effort involved to deploy and use this kind of sensing system,' Dr. Eickhoff notes.
The approach has numerous advantages. It is less complex, as only light is involved and there is no need for electrical contacts and measuring systems. The sensors require much lower operation temperatures compared to conventional sensor systems. And, because light - and not electricity - is all that is passing through the environment being monitored it is much safer, particularly in cases where the gas or liquid is flammable, pressurised or explosive.
EADS Innovation Works, a member of the Dotsense consortium, is interested in using opto-chemical sensor technology in aerospace applications, for example, where safety and robustness are major concerns.
'On an aircraft, they could be used to monitor water quality, hydraulic fluid, gas leaks or fuel,' Dr. Eickhoff notes. 'When we started the project, aeronautical applications were our main focus, but we soon realised that there are additional applications for this technology in many other industries.'
More sensitive than electrical sensors
Though the primary goal of the Dotsense project, supported by EUR 1.2 million in funding from the European Commission, was to develop chemical sensors that do not require electrical contacts, the team found that in several cases their all-optical solution is actually much more sensitive than electrical equivalents.
'The idea was not to make a highly sensitive device, but in the end it turned out that these optical nanostructures can actually be much more sensitive than electrical sensors,' Dr. Eickhoff says. 'It was certainly something we hoped might be the case, but we couldn't be certain until we conducted tests. Combined with their other advantages, that opens up a whole range of uses.'
He points, for example, to gas detection in industrial environments or home smoke alarms, to healthcare applications and to uses in the food processing industry to test the composition of liquids.
'There are many applications for these sorts of sensors. In fact, a lot of industrial sectors like the idea that you don't need electrical components and an electrical current running in the medium in which you are operating - the key benefits of this are safety and reliability,' the Dotsense coordinator explains.
Nonetheless, the technology remains some way away from commercial use. The Dotsense team overcame key technical challenges, such as pushing the emitted light from the transducers into the visible range so it can be excited by LEDs and detected with relatively inexpensive commercial photodetectors, controlling the growth of the nanostructures, and understanding the photo-electrical processes that occur on the surface of the nanostructures in different chemical environments. But more research is needed, Dr. Eickhoff notes.
Partly with that goal in mind, members of the team have launched a national follow-up project called 'Sinomics' in which they will integrate LEDs and photodetectors with nanostructures on-chip to develop innovative devices for gas sensing and detection.
'I'm optimistic that over the coming years this technology will find several applications, and it will become cheaper and hence more commercially viable to start producing all-optical sensors,' Dr. Eickhoff says.
Dotsense received research funding under the European Union's Seventh Framework Programme (FP7).