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Graphene is emerging as a powerful material for molecular sensors based on surface enhanced infrared absorption (SEIRA), as it exhibits mid-infrared (MIR) plasmonic tunability and extreme light confinement. While MIR probing of biomolecules - such as incubated proteins on graphene nanostructures – was successfully demonstrated in recent years, sensing of gas molecules can be challenging when relying on gas physisorption at the graphene surface. In this work, we employ an ultrathin gas-adsorbing polymer that optimizes gas sensing with graphene plasmons in an unprecedented combination. As a proof-of-concept, we used polyethylenimine (PEI) polymer deposited on top of graphene nanoribbons to selectively adsorb CO2 molecules. The ultrathin PEI layer concentrates the gas close (≤10 nm) to the graphene surface, so that the interaction with the plasmonic near field is significantly enhanced. Critical for the enhancement of graphene plasmon effect is the role of polymer-induced graphene doping. The varying CO2 concentrations can be transduced in changes in the surface optical response by both PEI vibrational mode enhancement and localized surface plasmon resonance (LSPR) modulation related to graphene chemical doping. The latter presents a novel and simpler transduction mechanism with respect to SEIRA effect. Also, we show that the optical response is reversible upon thermal desorption. The proposed hybrid gas sensor can be extended to different functional conductive polymer coatings that adsorb other relevant gases. Moreover, chemical-based doping of graphene plasmonic surfaces opens promising opportunities for gate-free graphene sensors.
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