Abstract
We introduce a manufacturable and scalable method for creating tunable wrinkled ferromagnetic-metallic structures to enhance fluorescence signals. Thin layers of nickel (Ni) and gold (Au) were deposited onto a pre-stressed thermoplastic (shrink wrap film) polymer. Heating briefly forced the metal films to buckle when the thermoplastic retracted, resulting in multi-scale composite 'wrinkles'. This is the first demonstration of leveraging the plasmons in such hybrid nanostructures by metal enhanced fluorescence (MEF) in the near-infrared wavelengths. We observed more than three orders of magnitude enhancement in the fluorescence signal of a single molecule of goat anti-mouse immunoglobulin G (IgG) antibody conjugated to fluorescein isothiocyanate, FITC, (FITC-IgG) by two-photon excitation with these structures. These large enhancements in the fluorescence signal at the nanoscale gaps between the composite wrinkles corresponded to shortened lifetimes due to localized surface plasmons. To characterize these structures, we combined fluctuation correlation spectroscopy (FCS), fluorescence lifetime imaging microscopy (FLIM), and two-photon microscopy to spatially and temporally map the hot spots with high resolution.
Highlights
A persistent challenge in fluorescence microscopy is to increase the signal to noise ratio of weakly fluorescent biomarkers or of biomolecules present at low concentration [1, 2]
We present here the first demonstration of leveraging the plasmons in Ni/Au composite structures for enhanced fluorescence signal by metal enhanced fluorescence (MEF) in the near-infrared wavelengths
Recent studies have shown that illuminating nanoscale metallic structures by a broadband femtosecond pulsed laser such as those used in two-photon microscopy can be an effective way to study the localization of light [46, 55]
Summary
A persistent challenge in fluorescence microscopy is to increase the signal to noise ratio of weakly fluorescent biomarkers or of biomolecules present at low concentration [1, 2] Strategies to increase both the quantum yield of the dye as well as to reduce the excitation volume have been demonstrated using nanoplasmonic structures such as gold (Au) bowtie. FCS revealed that the hot spots observed are reduced to below the confocal volume, and FLIM shows a shorter lifetime present on the composite structures than in solution This increase in the fluorescence signal and shortening in the fluorescence lifetime is due to the strong EM field localized around nanoscale gaps within our metallic structures. With this, we demonstrate we could detect single biomolecules by monitoring fluctuations in the fluorescence intensity of FITC-IgG antibody
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