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. 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.

In this work, we present an infrared plasmonic biosensor for chemical-specific detection and monitoring of biomimetic lipid membranes in a label-free and real-time fashion. Lipid membranes constitute the primary biological interface mediating cell signaling and interaction with drugs and pathogens. By exploiting the plasmonic field enhancement in the vicinity of engineered and surface-modified nanoantennas, the proposed biosensor is able to capture the vibrational fingerprints of lipid molecules and monitor in real time the formation kinetics of planar biomimetic membranes in aqueous environments. Furthermore, we show that this plasmonic biosensor features high-field enhancement extending over tens of nanometers away from the surface, matching the size of typical bioassays while preserving high sensitivity.

Plasmonic nanohole arrays have received significant attention, as they have highly advantageous optical properties for ultrasensitive and label-free biosensing applications. Currently, most of these subwavelength periodic apertures are mainly implemented on transparent materials, which results in multiple spectrally close transmission resonances. However, this spectral characteristic is not ideal for biosensing applications, as it complicates monitoring spectral variations. In this article, utilizing a hybrid substrate composed of a high refractive index dielectric interlayer over a transparent material, we show that gold nanohole arrays support spectrally isolated and well-defined plasmonic resonances that are easy to track. Compared to conventional configurations on transparent material, nanoholes on a hybrid substrate also exhibit plasmonic modes with well-preserved amplitudes, which is useful for reliable spectral monitoring. We show that nanohole arrays on a hybrid substrate are more sensitive to changes in surface conditions. Using a spectral integration method, which evaluates wavelength shifts in a large spectral window instead of monitoring only the plasmonic resonance wavelength, we obtain a detection limit as low as 2 × 10 −5 RIU. Furthermore, we successfully demonstrate real-time monitoring of biomolecular binding interactions even at sub-1 ng/mL levels.