Large Fluorescence Enhancements of Fluorophore Ensembles with Multilayer Plasmonic Substrates: Comparison of Theory and Experimental Results

Abstract

Multilayer substrates consisting of a glass slide, silver mirror, silica layer, and silver nanoparticles were fabricated using magnetron sputtering. This new geometry of substrates with backplane mirror and dielectric photonic cavity produced large average fluorescence enhancements up to 190-fold. Fluorescence enhancements of five fluorescent probes were measured over the broad spectral range from 470 to 800 nm. Fluorescent probes were streptavidin conjugates attached to the substrate surface through a layer of biotinylated bovine serum albumin. The protein layers represent a common surface modification for surface-based bioassays such as immunoassays or molecular diagnostic assays. We found that optimal enhancement is dependent on the thickness of the dielectric layer separating the silver mirror and the silver nanoparticles and on the spectral range. We performed numerical calculations for enhancement in both the excitation and emission using finite element method (FEM) the results of which were in qualitative agreement with the experimental results. The described method for fabrication multilayered substrates and the results obtained with protein layers demonstrate great potential for the design of simple and ultrasensitive fluorometric bioassays with large optical amplifications compared to the standard approaches of enzyme-based bioassays with dielectric surfaces.

A comparison of SERS and MEF of rhodamine 6G on a gold substrate

“…Surface Enhanced Raman Scattering (SERS) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and` Metal Enhanced Fluorescence (MEF) [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] both exploit the electric field enhancement associated with a plasmon resonance to substantially increase the spectroscopic signal. Despite the mechanistic similarities between SERS and MEF, there are few reports of SERS and MEF on the same structures.…”

Several hundred-fold enhanced fluorescence from single fluorophores assembled on silver nanoparticle–dielectric–metal substrate

“…We reported earlier measurements of several streptavidin conjugated organic dyes on PDM substrates showing enhanced emission for ensemble molecules up to 200-folds in the 450–800 nm wavelength range. 4 The single molecule studies of fluorophores at distance of ~10 nm from the metal surface is expected to provide more insights into the metal-fluorophore mechanism and a distribution of large enhancement depending on the location of the probe molecules. Current limitation for broader use of the SMS combined with plasmonic nanostructures is the need for precise placing of fluorophore in the near field.…”

“…We fabricated the PDM substrate as reported in our previous study 4a . Briefly, the silver thin layer nanoparticles were deposited over 50 nm thick silica layer which was deposited on 300 nm thick silver film.…”

“…Numerical calculations were performed based on the FEM using COMSOL Multiphysics as described in details. 4b …”

“…For numerical calculations an Ag array of 40 nm diameter particle and 40 nm edge-to-edge spacing was used that closely mimic the surface plasmon spectrum of Ag nanostructures of PDM substrates. 4b Average excitation and emission enhancements were calculated for a layer of 10 nm of water above the PDM surface with several of silica layers and compared to bare glass. The excitation enhancement for wavelength of 630 nm and 60 nm silica is shown in Figure 4C.…”