Plasmonic Vertically Coupled Complementary Antennas for Dual-Mode Infrared Molecule Sensing

Abstract

Here we report an infrared plasmonic nanosensor for label-free, sensitive, specific, and quantitative identification of nanometer-sized molecules. The device design is based on vertically coupled complementary antennas (VCCAs) with densely patterned hot-spots. The elevated metallic nanobars and complementary nanoslits in the substrate strongly couple at vertical nanogaps between them, resulting in dual-mode sensing dependent on the light polarization parallel or perpendicular to the nanobars. We demonstrate experimentally that a monolayer of octadecanethiol (ODT) molecules (thickness 2.5 nm) leads to significant antenna resonance wavelength shift over 136 nm in the parallel mode, corresponding to 7.5 nm for each carbon atom in the molecular chain or 54 nm for each nanometer in analyte thickness. Additionally, all four characteristic vibrational fingerprint signals, including the weak CH modes, are clearly delineated experimentally in both sensing modes. Such a dual-mode sensing with a broad wavelength design range (2.5 to 4.5 μm) is potentially useful for multianalyte detection. Additionally, we create a mathematical algorithm to design gold nanoparticles on VCCA sensors in simulation with their morphologies statistically identical to those in experiments and systematically investigate the impact of the nanoparticle morphology on the nanosensor performance. The nanoparticles form dense hot-spots, promote molecular adsorption, enhance near-field intensity 10 to 10 times, and improve ODT refractometric and fingerprint sensitivities. Our VCCA sensor structure offers a great design flexibility, dual-mode operation, and high detection sensitivity, making it feasible for broad applications from biomarker detection to environment monitoring and energy harvesting.

Engineering State‐of‐the‐Art Plasmonic Nanomaterials for SERS‐Based Clinical Liquid Biopsy Applications

“…For example, Ma et al developed a plasmonic silver "nanopore-in-nanogap" hybrid structure (Figure 9). The resulting "nanopore-innanogap" hybrid nanostructures thus possessed a combined nanopore and nanogap plasmon modes, which provided an additional electromagnetic enhancement for the ultrasensitive detection of 4-aminothiophenol down to 0.1 × 10 −15 m. Currently, there are many other hybrid nanostructures in reports, including a split-wedge antenna with 1 nm gaps, [71] vertically coupled complementary antenna, [72] bimetallic 3D Adv. An etching process was then performed to create ultrasmall nanopores of sub 10 nm in diameter on 2D silver nanoparticle supercrystals.…”

Leveraging of MEMS Technologies for Optical Metamaterials Applications

“…Therefore, quantifying the concentration of chemicals is also workable using surface enhanced spectroscopy. The detection limit of molecule concentration is shown to be as low as sub‐ppm level by the nanogap enhanced near‐field …”

“…The thin films of poly‐methyl methacrylate (PMMA) and ODT are the common analytes to characterize metamaterials sensing performance because they hold obvious fingerprints and are easy to functionalize on metamaterials resonator. During the last 10 years, many works were published utilizing various thin films of protein, lipid, DNA, and other biomolecular analytes .…”

“…Many fundamental works have been done to improve the sensitivity of plasmonic sensors. One approach is to increase the near‐field intensity by shrinking the gap between adjacent plasmonic nanoantennas to the nanometer scale, the other way is to expose the hotspot area for the access of molecules which is blocked by the substrate . However, there is a tradeoff between near‐field intensity and hotspot area in planar metamaterial resonators.…”

Exploiting Anisotropy of Plasmonic Nanostructures with Polarization Modulation Infrared Linear Dichroism Microscopy (µPM‐IRLD)

“…Achieving a single broad resonance that covers that entire range is incredibly difficult, thus alternative approaches are required to perform plasmon‐enhanced measurements. By exploiting the anisotropic response of nanostructures and metasurfaces, it is possible to overcome the need for a single‐broad resonance by instead generating a series of polarization‐dependent resonances . An advantage of this approach is that a given resonance or set of resonances can be individually tuned to a specific frequency, or frequencies, and therefore individually excited with a given polarization.…”