Wednesday, 14 September, 2016
Tiny measuring device for humidity and solvent vapors
NIM scientists developed a measuring device which is only a few nanometers in size but fulfills two functions: it detects very low humidity levels and identifies vapors of organic solvents. The central part of the device is a stack of nanolayers.
If viewed under the electron microscope, the research project looks like a piece of puff pastry: a handful of loosely stacked nanometer-thin layers of antimony phosphate. “What we have here is in fact a tiny and extremely sensitive measuring instrument which offers two functions. It can detect traces of water and, in addition, visually differentiate organic solvents,” explains PhD student Katalin Szendrei. “The water or solvent vapor diffuses between the nanolayers, thereby changing their conductivity or color within seconds.” The chemist works in the group of NIM Professor Bettina Lotsch. Together with her colleague Pirmin Ganter she is lead author of the accompanying publication in the journal “Advanced Materials”. The characteristics of the new nano-sized measuring device sound promising: fast, highly sensitive and precise measurements, easy handling. And all this in a tiny device which is so small that it could be installed everywhere.
Changes in conductivity
Professor Bettina Lotsch explores the nanolayers both with her group “Functional nanostructures” at LMU Munich and with her team at the Max Planck Institute for Solid State Research in Stuttgart. “We developed two types of these thin-layer films,” she explains. “The first construction is composed of a stack of nanolayers to which we attached electrodes. The higher the humidity is and the more water molecules are deposited between them, the more conductivity increases. The measurement is so sensitive that we can even detect a moisture degree of 0.2 percent.”
Detection by color change
The second detection method is based on the fact that volatile analytes influence the thickness of the nanolayer stacks and thus the refraction of light, that is to say, the color. Besides the detection of water, this method allows scientists to visually differentiate the vapors of common solvents such as alcohols, acetonitrile, toluene or heptane. The system even distinguishes chemically similar substances like the constitutional isomers 1-propanol and 2-propanol. The chemists take several stacks and put titanium dioxide nanoparticles in between. These nanoparticles ensure a well-defined periodic arrangement of the stacks. The scientists then put this hybrid structure in a chamber where they expose it to water or solvent vapor. The molecules diffuse into the nanolayer stacks causing them to swell up. In addition, they settle in the free pores in the nanoparticle layer.
Depending on the swelling degree, light with different wavelengths is reflected and interfered at the layer transitions. This results in different structural colorations such as we know from butterfly wings, opals or mussel shells. “By using these nanolayer-based photonic crystals we are the first to visually differentiate solvent vapors with extremely high sensitivity using a single-component sensor,” explains Katalin Szendrei. “Other approaches require a combination of up to nine sensors and their statistical evaluation is time-consuming.”
It will surely take some more time until a measuring device based on this new sensor is available, but market players have already signaled their interest.
Humidity Sensing: Towards the Nanosheet-Based Photonic Nose: Vapor Recognition and Trace Water Sensing with Antimony Phosphate Thin Film Devices. Pirmin Ganter, Katalin Szendrei, Bettina V. Lotsch. Advanced Materials, Volume 28, Issue 34, September 14, 2016 , Page 7294
Prof. Bettina Lotsch
Ludwig-Maximilians-Universität München (LMU)
Butenandstr. 5-13 (Haus D)
Tel: +49 (0)89 2180-77429