Magnetism + light = supersensors

A happy marriage between magnetism and light brought the research of Alexander Dmitriev to a new level. The concept of extremely sensitive biological and chemical sensors, able to detect very small molecules – the elementary building blocks for large groups of polymers, peptides and proteins – has now been tested through collaboration with research groups at nanoGUNE, San Sebastian, Spain and the University of Gothenburg. The result was recently presented in a paper in Nature Communications.

—   It’s fantastic! For a researcher there is typically a long way between a concept and its realisation, but now we actually got to see how it worked, says Alexander Dmitriev, Associate Professor at the Department of Applied Physics at Chalmers.

During the past fifteen years Chalmers has built a strong competence within the area of label-free, optical biological and chemical sensors. The scientists have focused on nanostructures that are able to capture light via so-called ‘localized plasmons’, collective oscillations of electrons, following the same principle as for stained glass windows used in old churches, where light passing through the glass is filtered in different colours. The important feature of localized plasmons is that they are very sensitive to the immediate surrounding of the nanostructures, so they can report the presence of very small amounts of, for example, proteins that land on them from a biological solution. The metals used in such nanostructures as antennas for light are usually gold or silver. To couple light effectively into localized plasmons with ferromagnetic metals like nickel or cobalt was for a long time considered in practice impossible.

An unexpected mix
However when Dmitriev, several years back, got to know Johan Åkerman, Professor of Experimental Physics at the University of Gothenburg, they started to build on the idea of bringing light and magnetism together at the nanoscale using plasmons.

—   If you are open to new ideas and other areas of research you will always find unexpected ways to bring your research forward, says Dmitriev. Johan Åkerman and Prof. Paolo Vavassori of nanoGUNE are experts in nanomagnetism and here at Chalmers we are prominent in nanoplasmonics. Together we created a good mix that works.

What they did was to use nickel, instead of gold or silver, in order to combine magnetic fields and light for a new way of optical sensing, using magneto-optical Kerr effect. This effect allows rotating the polarization of light in ferromagnets by the applied magnetic field. Adding plasmons to the mixture makes this rotation strongly enhanced, and largely improves our ability to detect how plasmons sense molecules, even very small ones.

The concept put to test
In 2011 Dmitriev, Åkerman and Vavassori published the first articles showing the concept for a new ‘magnetoplasmonic’ biosensor. Taking it a step further, they have now shown a striking 100-fold higher sensor response as compared to other sensors based on localized plasmons – one-molecule-thick layer at a time.

—   Here we brought in what we do best, which is the sensor platform – a glass surface with ferromagnetic nanoantennas. The group of Professor Mato Knez in San Sebastian are among the best in the world at molecular layer deposition – a very controlled way of building molecular layers. Being able to show different sensor responses depending on how many extremely thin molecular layers we have added, we can confidently say that this method is among the best there is in optical label-free sensing right now.

The discovery is primarily directed towards biomedicine and diagnostics as an efficient way to retrieve more information from a smaller amount of fluids, like blood or urine. But according to Alexander Dmitriev the method could also be applied to other areas such as organic solar cells.

—   It’s now open to other researchers to build upon this proof-of-concept advance and to create practical devices that can put these ideas to use.

Text and photo: Karin Weijdgård