Lab-on-a-chip tests chemical composition of liquids

Researchers at the Vienna University of Technology (TU Wein) have taken a technique for measuring the chemical composition of liquids and implemented it in a tiny sensor. Their work has been published in the journal Nature Communications.

Just one drop is enough to test the chemical composition. Copyright: TU Wien.

Quantum cascade lasers are made of a perfectly optimised layer system of different materials. That way, properties such as the wavelength of the laser can be tuned. When voltage is applied to the layer structure, the laser starts to emit light. But the structure can also work the other way around; when it is irradiated with light, an electric signal is created.

Infrared laser beams are well suited for analysing liquids and gases, as they are absorbed differently by different molecules. This effect can, for instance, be used to measure the oxygen concentration in blood. Now a method has been developed to create a laser and a detector at the same time, on one single chip, in such a way that the wavelength of the laser perfectly matches the wavelength to which the detector is sensitive.

In conventional systems, the laser light has to either be transmitted to the detector using carefully placed lenses, or via optical fibres, but they usually transport all the light inside and therefore they cannot be used as sensors. The researchers’ optical connection between quantum cascade laser and detector works in a completely different way, with a gap of 50 µm bridged by a plasmonic waveguide made of gold and silicon nitride.

The light is emitted by the laser (top), transported on the plasmonic waveguide (blue) and hits the detector (bottom right). How much of it is absorbed depends on the chemical composition of the liquid. Copyright: TU Wien.

“The light interacts with the electrons in the metal in a very special way, so that the light is guided outside the gold surface,” said PhD student Benedikt Schwarz. “That is why the light can be absorbed by the molecules on its way between laser and detector.”

The sensor chip can be submerged in liquid, and by measuring the decrease of the detected light intensity due to the presence of light-absorbing molecules, the composition of the liquid can be determined. The sensor was tested with a mixture of water and alcohol, and the water concentration was measured with an accuracy of 0.06%.

As the wavelength can be influenced by changing the design of the layered structure, the sensor concept can be applied to a wide variety of molecules, such as carbohydrates or proteins, for many applications in chemical, biological or medical analytics.

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