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Graphene used to improve infrared absorption spectroscopy

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Researchers at the École Polytechnique Fédérale de Lausanne (EPFL; Switzerland) and Instituto de Ciencias Fotónicas (ICFO; Spain) have used graphene’s unique optical and electronic properties to develop a highly sensitive molecular sensor. The sensor could be used to detect molecules such as proteins and drugs. This work was recently published in Science.

The researchers used graphene to improve infrared absorption spectroscopy, which is not effective in the detection of nanometrically-sized molecules. The wavelength of the infrared photon is approximately 6 microns, while the target measures only a few nanometers.

Graphene can be used for the difficult task of measuring nanometrically-sized molecules as—given the correct geometry—graphene can focus the light on a precise spot on its surface, allowing for the detection of the vibration of an attached nanometric molecule.

“We first pattern nanostructures on the graphene surface by bombarding it with electron beams and etching it with oxygen ions,” said Daniel Rodrigo, Researcher at EPFL and co‐author of the publication. “When the light arrives, the electrons in graphene nanostructures begin to oscillate. This phenomenon, known as ‘localized surface plasmon resonance’, serves to concentrate light into tiny spots, which are comparable with the dimensions of the target molecules. It is then possible to detect nanometric structures.”

The nature of the bonds connecting the atoms that the molecule comprises can also be discovered from the range of vibrations generated by the bonds. In order to pick up vibrations from each bond, the graphene can be ‘tuned’ to different wavelengths by applying voltage (this is not possible with currently available sensors). Making graphene’s electrons oscillate in different ways makes it possible to detect 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,” noted Hatice Altug, Associate Professor at EPFL and corresponding author for the study.

A key advantage of the device is that it allows for the conduction of a complex analysis using only one device, whereas it would usually require many different devices. Furthermore, there is no modification of the biological sample.

This device demonstrates graphene’s enormous potential in the field of detection. “There are many possible applications,” said Altug. “We focussed on biomolecules, but the method should also work for polymers, and many other substances.”

Sources: A graphene‐based sensor that is tunable and highly sensitive; Rodrigo D, Limaj O, Janner D et al. Mid-infrared plasmonic biosensing with graphene. Science 349, 165—168 (2015).

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