Abstract
Plasmonic biosensors, operating in the mid-infrared (mid-IR) region, are well-suited for highly specific and label-free optical biosensing. The principle of operation is based on detecting the shift in resonance wavelength caused by the interaction of biomolecules with the surrounding medium. However, metallic plasmonic biosensors suffer from poor signal transduction and high optical losses in the mid-IR range, leading to low sensitivity. Here, we introduce a hyperbolic metamaterial (HMM) biosensor, that exploits the strong, tunable, mid-IR localization of graphene plasmons, for detecting nanometric biomolecules with high sensitivity. The HMM stack consists of alternating graphene/Al2O3 multilayers, on top of a gold grating structure with rounded corners, to produce plasmonic hotspots and enhance sensing performance. Sensitivity and figure-of-merit (FOM) can be systematically tuned, by varying the structural parameters of the HMM stack and the doping levels (Fermi energy) in graphene. Finite-difference time-domain (FDTD) analysis demonstrates that the proposed biosensor can achieve sensitivities as high as 4052 nm RIU−1 (refractive index unit) with a FOM of 11.44 RIU−1. We anticipate that the reported graphene/Al2O3 HMM device will find potential application as a mid-IR, highly sensitive plasmonic biosensor, for tunable and label-free detection.
Highlights
Plasmonic biosensors have been extensively used for rapid, realtime, and label-free detection of biomolecules, at ultralow concentrations.[1,2] These devices, primarily targeted for point-ofcare (POC) applications, rely on the excitation of coherent oscillations of delocalized conduction band electrons, when light, incident on a metal/dielectric interface, meets the desired resonance conditions.[3]
In the detection limit, is evaluated in terms of the gure of merit (FOM) which is expressed as the ratio of sensitivity to the full width at half-maximum (FWHM) of the resonance dip.[5]
Owing to the atomically thin nature of graphene, hyperbolic metamaterial (HMM) sensors based on graphene can offer extreme scalability, which is important in realizing portable, POC biosensors
Summary
Conventional, state-of-the-art plasmonic biosensors make use of metallic nanostructures to con ne the electric eld within 5–15 nm of the nanostructure surface, giving rise to near eld enhancements through LSPR.[6]. Hong et al combined asymmetric gold (Au) nano-antennas and unpatterned graphene sheets to achieve multi-functional, broadband sensing, covering both near-IR and mid-IR wavelengths.[16] Optical conductivity based mid-IR sensors, with ultrahigh sensitivity, was reported by Zhu et al through the use of Au nanorod antenna array covered by monolayer graphene.[29] the study by Wu et al employed a graphene sheet, integrated on top of a Au grating structure, for mid-IR sensing of vibrational modes from protein molecules.[20]. Each metal/dielectric bilayer evanescently couples the short range, propagating SPPs to its adjacent bilayer, leading to the existence of BPPs. Replacing the metal with graphene, can lead to new possibilities in terms of stronger plasmon response in mid-IR range, smaller material loss and tunable conductivity.[36,37,38] owing to the atomically thin nature of graphene, HMM sensors based on graphene can offer extreme scalability, which is important in realizing portable, POC biosensors. The sensor performance is numerically evaluated, in terms of both sensitivity and FOM, for a wide range of RIs that correspond to commonly used biomolecules
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