  Infrared spectroscopy is the technique of choice for chemical identification
of biomolecules through their vibrational fingerprints. However, infrared light
interacts poorly with nanometric size molecules. Here, we exploit the unique
electro-optical properties of graphene to demonstrate a high-sensitivity
tunable plasmonic biosensor for chemically-specific label-free detection of
protein monolayers. The plasmon resonance of nanostructured graphene is
dynamically tuned to selectively probe the protein at different frequencies and
extract its complex refractive index. Additionally, the extreme spatial light
confinement in graphene, up to two orders of magnitude higher than in metals,
produces an unprecedentedly high overlap with nanometric biomolecules, enabling
superior sensitivity in the detection of their refractive index and vibrational
fingerprints. The combination of tunable spectral selectivity and enhanced
sensitivity of graphene opens exciting prospects for biosensing.
