Detecting molecules using improved infrared absorption spectroscopy based on grapheneA new study has produced a reconfigurable and very sensitive molecule sensor by manipulating the optical and electronic properties of graphene. A team from the École Polytechnique Fédérale De Lausanne and the Institute of Photonic Sciences in Spain used graphene to make improvements to infrared absorption spectroscopy, a common technique for detecting molecules.
Although light is normally used to excite molecules, which vibrate differently depending on their nature, this approach is impractical for detecting nanometrically sized molecules. However, with the right geometry, graphene can focus the light on a specific area on its surface and pick up the vibration of a nanometric molecule attached to it. The team patterned nanostructures on the graphene surface by bombarding it with electron beams before etching it with oxygen ions. When the light arrives, electrons in the graphene nanostructures start to oscillate, a phenomenon known as 'localized surface plasmon resonance'. This focuses light into tiny spots that are comparable to the dimensions of the target molecules, helping to detect nanometric structures.
"this new level of light confinement and the dynamical tunability of graphene offers great opportunities for infrared biosensing”Hatice Altug
The process can also determine the nature of the bonds connecting the atoms that the molecule is made up of. When a molecule vibrates, it produces a range of vibrations that are generated by the bonds connecting the different atoms. Each vibration can be identified by nuances that provide information on the nature of each bond as well as the health of the whole molecule, acting as a fingerprint for identifying the molecule.
The researchers, whose work was published in Science [Rodrigo et al. Science (2015) DOI: 10.1126/science.aab2051], ‘tuned’ the graphene to different frequencies by applying voltage. With graphene's electrons oscillating differently, it is possible to ‘read’ all the vibrations of the molecule on its surface. The method demonstrates how to carry out complex analysis with one device rather than many, and with no stress or modification of the biological sample, highlighting graphene's potential in the field of detection.
Combining tunable spectral selectivity with enhanced sensitivity of graphene could lead to many applications, especially as the sensor detects molecular vibrations in the infrared range, which are found for practically any material. As researcher Hatice Altug points out, “We believe that this new level of light confinement and the dynamical tunability of graphene offer great opportunities for infrared biosensing.”
The sensor could also be suitable for applications involving non-destructive tests to distinguish between materials of a different chemical nature, such as in clinics and diagnostics, biotechnology, material science, food safety, pharmaceutics and environmental monitoring.