Science 349, 165–168 (2015)

Mid-infrared vibrational modes of biomolecules can be used for label-free sensing, but the sensitivity of this approach is undermined by the fact that the wavelength of light (2–6 μm) is much larger than the size of the biomolecules (less than 10 nm). One way to overcome this fundamental hurdle is to confine light to nanometre-scale volumes using localized surface plasmons. Light confinement in metals, however, leads to reduced spectral bandwidth, which is determined by the geometry of the system. Hatice Altug and colleagues at the École Polytechnique Fédérale de Lausanne, the Institut de Ciències Fotònique in Barcelona and Institució Catalana de Recerca i Estudis Avançats in Barcelona have now shown that some of these constraints can be relaxed by using graphene as a mid-infrared biosensor for proteins.

The researchers fabricated an array of 30-nm-wide graphene nanoribbons on top of a silica support and connected the ribbons to a gate electrode. They then shone a mid-infrared laser beam to excite the graphene plasmon resonance. By changing the voltage bias, Altug and colleagues could modulate the plasmon resonance so that it overlaps with specific vibrational frequencies. When they added a protein layer on top of the device, the C=O stretch and N–H bend modes at 1,660 and 1,550 cm−1, respectively, coupled with the graphene plasmon resonance. As a result, the extinction spectrum showed distinct dips. Because graphene produces tighter light confinement than metals, the approach also has higher sensitivity than metallic devices working in the same spectral range.