Optical sensor offers greater sensitivity to dangerous chemicals

May 17, 2012 // By Paul Buckley
A team of EU-funded researchers has developed an optical solution that claims to be safer, easier to install and, more sensitive than electrical sensors.

Developed as part of the Dotsense project, the sensors are based on an innovative application of quantum dots and nanowires. Made of the group III-nitride semiconductor system - (Al,In)GaN - chemically stable semiconductor materials with excellent opto-electronic properties, the 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,” explained 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,” explained Dr. Eickhoff.     

The approach has numerous advantages. It is less complex, as only light is involved and there is no need for electrical