Chemical sensor, integrated on a chip
Austrian researchers succeeded in manufacturing a prototype of a sensor-on-a-chip: They created quantum cascade laser sources and laser detectors on the same chip, with the distance between laser and detector of just 50µm. This gap is bridged by a surface plasmon wave guide made of gold and silicon nitride. This approach is new and enables simple, cost-effective production of miniaturised liquid sensors for a variety of applications.
Unlike normal laser sources, quantum cascade laser sources consist of a perfectly optimised sequence of layers of different materials. Thus, designers and scientists can control relevant properties and a laser source, for example its wavelength. If an external voltage is applied to this layered structure, the laser source starts to emit light. It also works the other way around: If irradiated with light, they generate an electric signal.
Scientists from the Vienna Technical University have developed a method to create a laser source and a sensor with identical layered structures on a single chip in one process. The wavelength of the source is the same as the wavelength of the sensor. This bi-functional material is produced at the Centre of micro and nano structures of the Vienna Technical University by means of molecular expitaxy – atomic layer by atomic layer.
In conventional systems, the laser beam has to be directed to the detector by means of very precisely adjusted lenses. Fibre optics can be used also, but they typically transport the light inside themselves, the light cannot be coupled out and for this reason fibre optics are inappropriate for use as sensors.
The new device introduced by the Vienna Technical University features a completely different optical connection between quantum cascade laser source and detector: It is a plasmonic waveguide consisting of gold and silicon nitride. The light interacts with the electrons of the metal in a very specific way, directing the light to the outside of the gold surface. Thus, the light can be absorbed by molecules on its way from the laser source to the detector.
If the chip is submerged into a liquid, the attenuation of the light signal, caused by absorbing molecules, allows to draw conclusions with regard to the composition of the liquid. The research team tested the function of the chip with am mixture of water and alcohol. The result: It allows measurement accuracies of 0.06%. Since the wavelength of the sensor can be determined by the layer sequence, this concept can be applied to a broad range of molecules such as hydrocarbons or even proteins for many different applications in chemical, biological and medical analysis.
For further information contact benedict.schwarz@tuwien.ac.at
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