Abstract
Enhancement of optical emission on plasmonic nanostructures is intrinsically limited by the distance between the emitter and nanostructure surface, owing to a tightly-confined and exponentially-decaying electromagnetic field. This fundamental limitation prevents efficient application of plasmonic fluorescence enhancement for diversely-sized molecular assemblies. We demonstrate a three-dimensionally-tapered gap plasmon nanocavity that overcomes this fundamental limitation through near-homogeneous yet powerful volumetric confinement of electromagnetic field inside an open-access nanotip. The 3D-tapered device provides fluorescence enhancement factors close to 2200 uniformly for various molecular assemblies ranging from few angstroms to 20 nanometers in size. Furthermore, our nanostructure allows detection of low concentration (10 pM) biomarkers as well as specific capture of single antibody molecules at the nanocavity tip for high resolution molecular binding analysis. Overcoming molecule position-derived large variations in plasmonic enhancement can propel widespread application of this technique for sensitive detection and analysis of complex molecular assemblies at or near single molecule resolution.
Highlights
Enhancement of optical emission on plasmonic nanostructures is intrinsically limited by the distance between the emitter and nanostructure surface, owing to a tightly-confined and exponentially-decaying electromagnetic field
Our analysis reveals that the 3D-taper geometry improves the coupling of molecular emitters to the electromagnetic field, delivering up to 28% improvement in the radiative decay rates and leading to even stronger enhancement of fluorescence
We have demonstrated 3D-tapered gap plasmon nanocavities, which provide one of the highest enhancement of fluorescence obtained by plasmonic nanostructures (EF: ~2200 with figure of merit ~260), independent of the size of the molecular assemblies used in the assay
Summary
Enhancement of optical emission on plasmonic nanostructures is intrinsically limited by the distance between the emitter and nanostructure surface, owing to a tightly-confined and exponentially-decaying electromagnetic field This fundamental limitation prevents efficient application of plasmonic fluorescence enhancement for diversely-sized molecular assemblies. Fluorescence enhancement obtained from plasmonic nanostructures is intrinsically dependent on both the efficiency of electromagnetic (EM) field confinement at the plasmonic hotspot as well as the distance between the optical emitter (fluorophore) and the plasmonic hotspot[8,9,10,11,12] These fluorophores are typically attached to biorecognition elements such as antibodies or nucleic acid aptamers that recognize and bind to other target molecules. Powerful fluorescence enhancement within a nanostructure independent of variation in molecule size and position can be expected to rely on several important factors: (a) strong confinement of electromagnetic field (b) powerful coupling of the emitter to the field for enhancement of emission and (c) a hotspot geometry, which generates uniform electromagnetic field distribution. Overcoming the molecule placement limitation for plasmonic enhancement of fluorescence such as presented in this manuscript can allow this technique to be reliably and widely applicable for a broad range of biological assays including complex molecular assemblies
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