Many areas of fundamental research are interested in graphene owing to its exceptional characteristics. It is made of one layer of carbon atoms, which makes it light and sturdy, and it is an excellent thermal and electrical conductor. Its unique features make it potentially suitable for applications in a number of areas . Scientists at EPFL's Bionanophotonic Systems Laboratory (BIOS) together with researchers from ICFO- The Institute of Photonic Sciences in Barcelona, have now harnessed graphene's unique optical and electronic properties to develop a reconfigurable highly sensitive molecule sensor. The results are described in an article appearing in the latest edition of the journal Science.
Focussing light to improve sensing
The researchers used graphene to improve on a well-known molecule-detection method: infrared absorption spectroscopy. In the standard method, light is used to excite the molecules, which vibrate differently depending on their nature. It can be compared to a guitar string, which makes different sounds depending on its length. By virtue of this vibration, the molecules reveal their presence and even their identity. This "signature" can be "read" in the reflected light. This method is not effective, however, in detecting nanometrically-sized molecules. The wavelength of the infrared photon directed at a molecule is around 6 microns (6,000 nanometres), while the target measures only a few nanometres. It is very challenging to detect the vibration of such a small molecule in reflected light.
This is where graphene comes in. If given the correct geometry, graphene is capable of focussing light on a precise spot on its surface and "hearing" the vibration of a nanometric molecule that is attached to it. In this study, researchers first pattern nanostructures on the graphene surface by bombarding it with electron beams and etching it with oxygen ions. When the light arrives, the electrons in graphene nanostructures begin to oscillate. This phenomenon concentrates light into tiny spots, which are comparable with the dimensions of the target molecules. It is then possible to detect nanometric compounds in proximity to the surface.
From ICFO, focussing on future industrial applications of this new sensor, Prof. Valerio Pruneri commented that "the concept can be used in different application fields, ranging from gas leakage, toxic and explosive gas sensing, and contaminants in water to DNA and proteins. This is because graphene is an inert material for the elements to be detected and the reading mechanism uses light which is free of any interference effect". "The beauty of this material lies in its simplicity of structure, which translates in an equally simple electro-optic response", adds ICFO Prof. Javier García de Abajo who contributed by first demonstrating theoretically its behaviour .
Reconfiguring graphene in real time to see the molecule's structure
In addition to identifying the presence of nanometric molecules, this process can also reveal the nature of the bonds connecting the atoms that make up the molecule. Graphene is able to pick up the sound given off by each of the strings because it is able to identify a whole range of frequencies. Researchers "tuned" the graphene to different frequencies by applying voltage, which is not possible with current sensors. Making graphene's electrons oscillate in different ways makes it possible to "read" all the vibrations of the molecule on its surface. "We tested this method on proteins that we attached to the graphene. It gave us a full picture of the molecule," said Hatice Altug.
A big step closer to using graphene for molecule sensing
The new graphene-based process represents a major step forward for the researchers, for several reasons. First, this simple method shows that it is possible to conduct a complex analysis using only one device, while it normally requires many different ones. And all this without stressing or modifying the biological sample. Second, it stresses graphene's incredible potential in the area of sensing.
Daniel Rodrigo, Odeta Limaj, Davide Janner, Dordaneh Etezadi, F. Javier García De Abajo, Valerio Pruneri, Hatice Altug Mid-Infrared Plasmonic Biosensing With Graphene, Science
The article in Science: http://www.
EPFL BioNanoPhotonic Systems Laboratory BIOS: http://bios.
ICFO-The Institute of Photonic Sciences: https:/
Research group led by Prof. Javier Garcia de Abajo: https:/
Research group led by Prof. Valerio Pruneri: https:/