Fluorescence Enhancement via Dual Coupling of Dye Molecules with Silver Nanostructures

Vien Thi Tran,Heongkyu Ju

doi:10.3390/chemosensors9080217

Abstract

We demonstrate the enhancement of fluorescence emitted from dye molecules coupled with two surface plasmons, i.e., silver nanoparticles (AgNPs)-induced localized surface plasmons (LSP) and thin silver (Ag) film supported surface plasmons. Excitation light is illuminated to a SiO2 layer that contains both rhodamine 110 molecules and AgNPs. AgNPs enhances excitation rates of dye molecules in their close proximity due to LSP-induced enhancement of local electromagnetic fields at dye excitation wavelengths. Moreover, the SiO2 layer on one surface of which a 50 nm-thick Ag film is coated for metal cladding (air on the other surface), acts as a waveguide core at the dye emission wavelengths. The Ag film induces the surface plasmons which couple with the waveguide modes, resulting in a waveguide-modulated version of surface plasmon coupled emission (SPCE) for different SiO2 thicknesses in a reverse Kretschmann configuration. We find that varying the SiO2 thickness modulates the fluorescent signal of SPCE, its modulation behavior being in agreement with the theoretical simulation of thickness dependent properties of the coupled plasmon waveguide resonance. This enables optimization engineering of the waveguide structure for enhancement of fluorescent signals. The combination of LSP enhanced dye excitation and the waveguide-modulated version of SPCE may offer chances of enhancing fluorescent signals for a highly sensitive fluorescent assay of biomedical and chemical substances.

Highlights

Plasmonic metal nanostructures have been intensively explored due to their intrinsic features of the local fields enhancement for strengthened interaction between light and electronic dipoles that scatter or fluoresce
We report an approach of combining localized surface plasmons (LSP)-induced enhanced excitation of dye molecules and the waveguide-surface plasmon coupling at dye emission wavelengths for waveguide-modulated surface plasmon coupled emission (SPCE)
We demonstrate the enhancement of fluorescence signal emitting from rhodamine

Summary

Introduction

Plasmonic metal nanostructures have been intensively explored due to their intrinsic features of the local fields enhancement for strengthened interaction between light and electronic dipoles that scatter or fluoresce. Fluorescence, in general, inherently suffers from photo-bleaching and restricted efficiency in collecting all-direction radiated fluorescent light, but these issues can be circumvented via tailoring of plasmonic local fields around dye molecules Metal nanostructures such as thin metal films can support the surface plasmon coupled emission (SPCE) of fluorophores that occurs via the non-radiative decay channel of the excited dye molecules into surface plasmons through near field interaction in a reverse Kretschmann configuration [18,20,21]. We find that the thickness dependence of the fluorescent signal of SPCE qualitatively follows that of the depth-to-width ratio (Γ) of the reflectance dip in a Kretschmann configuration that uses the SiO2 layer for CPWR Considering such Γ as proportional to quality factor of surface plasmons [37], this feature enables the SiO2 thickness-dependent fluorescent signal of SPCE to be interpreted as due largely to thicknessdependent interaction of excited dye molecules with surface plasmons whose evanescent fields strength into the SiO2 layer is thickness-dependent at λem. Combining an LSP-induced increase in excitation rate and waveguide-enhanced SPCE for fluorescent signal optimization may find use in fluorescence spectroscopy for a highly sensitive assay of biomedical and chemical substances

Fabrication of the Plasmonic Chip for Fluorescence Enhancement

Fluorescence Detection System

Findings

Result and Discussion

Conclusions