Hybrid Plasmonic Nanostructures for Enhanced Single-Molecule Detection Sensitivity

Abstract

Biosensing applications based on fluorescence detection often require single-molecule sensitivity in the presence of strong background signals. Plasmonic nanoantennas are particularly suitable for these tasks, as they can confine and enhance light in volumes far below the diffraction limit. The recently introduced antenna-in-box (AiB) platforms achieved high single-molecule detection sensitivity at high fluorophore concentrations by placing gold nanoantennas in a gold aperture. However, hybrid AiB platforms with alternative aperture materials such as aluminum promise superior performance by providing better background screening. Here, we report on the fabrication and optical characterization of hybrid AiBs made of gold and aluminum for enhanced single-molecule detection sensitivity. We computationally optimize the optical properties of AiBs by controlling their geometry and materials and find that hybrid nanostructures not only improve signal-to-background ratios but also provide additional excitation intensity and fluorescence enhancements. We further establish a two-step electron beam lithography process to fabricate hybrid material AiB arrays with high reproducibility and experimentally validate the higher excitation and emission enhancements of the hybrid nanostructures as compared to their gold counterpart. We foresee that biosensors based on hybrid AiBs will provide improved sensitivity beyond the capabilities of current nanophotonic sensors for a plethora of biosensing applications ranging from multicolor fluorescence detection to label-free vibrational spectroscopy.

Hexagonal Plasmonic Arrays for High-Throughput Multicolor Single-Molecule Studies

“…The excitation intensity enhancement shows a complex modal structure originating from the interplay of the BNA plasmon resonance, the NH cavity modes, and lattice modes from the periodic hexagonal packing. Similar to previous findings, we find that HCP-AiBs provide excitation intensities far above the ones of NHs and even BNAs mostly due to the coupling of the NH cavity mode to the BNA plasmon resonance . Furthermore, we observe a red shift of the resonances for increasing radii for the three plasmonic systems under study.…”

“…Figure (a) shows scanning electron microscope (SEM) images of a 32 × 32 HCP-AiB array fabricated with a three-step EBL overlay process that is detailed in Figure S3 and associated text of the Supporting Information and derived from an EBL process we reported earlier . The geometrical parameters of the fabricated HCP-AiBs agree very well with the computationally optimized parameters, except for a slight ∼10% increase of r that is still within a good range for multicolor applications.…”

“…Based on these considerations, we opted for HCP-AiBs consisting of aluminum bowtie nanoantennas (BNAs) within aluminum nanoholes (NHs). Aluminum is our material of choice as it provides broadband plasmonic resonances throughout the visible wavelength range and naturally forms a thin protective oxide layer of about 3 nm. − Furthermore, we showed in a recent study that AiBs with an aluminum nanoaperture provide superior fluorescence emission rate enhancement and signal-to-background ratios (SBRs) . Given the round AiB shape, an HCP arrangement is chosen to ensure an optimal packing density.…”

“…The optimal radius r is determined in a more elaborate computational optimization as it has a significant influence on both the fluorescence emission rate and SBR . Moreover, it is the only parameter that influences the center-to-center distance 2· R – given that Δr is fixed by the diffraction limit–and therefore affects lattice effects originating from the periodic hexagonal packing.…”

“…To overcome these challenges, Punj et al introduced antenna-in-box (AiB) platforms consisting of a nanoantenna within a nanoaperture and thus unifying strong fluorescence background reduction and fluorescence emission enhancement . Throughout the years, AiB designs have been further refined toward higher single-molecule detection sensitivity and biocompatibility and adapted by various authors for biosensing applications with model and living cell membranes. − Despite providing single-molecule sensitivity at concentrations above 20 μM, the wider adaptation of AiBs for biosensing applications is currently hampered by two key reasons. First, current AiB designs are limited to single-color fluorescence applications in the red and near-infrared wavelength range due to the use of gold as plasmonic material .…”