Quantum dot based chemical sensors are safer, smaller, more sensitive

Giessen, Germany--European Union funded researchers have developed an optical chemical sensor that is safer, easier to install and much more sensitive than most existing chemical sensors.

Developed as part of the Dotsense project, the sensors are based on quantum dots and nanowires. Made of the chemically stable group III nitride semiconductor system aluminum indium gallium nitride (AlInGaN), 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," says. Martin Eickhoff, the Dotsense project coordinator at Justus-Liebig-University. "With our approach, that's unnecessary. Our solution is based on a completely optical analysis."

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.

"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," Eickhoff notes.

The sensors require much lower operation temperatures compared to conventional sensor systems. And, because light is all that is passing through the environment being monitored, it is much safer, particularly in cases where the gas or liquid is flammable, pressurized, or explosive.

EADS Innovation Works, a member of the Dotsense consortium, is interested in using optochemical 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," Eickhoff notes.

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.

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 photoelectric processes that occur on the surface of the nanostructures in different chemical environments. But more research is needed, 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 devices for gas sensing and detection.