UV to NIR Surface Plasmon Coupled and Metal-Enhanced Fluorescence Using Indium Thin Films: Application to Intrinsic (Label-less) Protein Fluorescence Detection

Surface plasmon coupled fluorescence (SPCF) and metal enhanced fluorescence (MEF) over a broad wavelength range from UV to NIR are demonstrated from 20 nm thin indium films. Steady-state and time-resolved fluorescence measurements of fluorophores deposited on the indium thin films indicate enhancement of the fluorescence intensity along with decreases in decay times, both in MEF and SPCF modes, accompanied by an increase in photostability. MEF was found to be more pronounced for the red emission dye with respect to a blue emission dye. A completely p -polarized directional emission for three fluorophores from a hemicylindrical prism at 44° indicates surface plasmons couple with the near field dipoles at the interface of the indium-sample. The evanescent waves generated at the interface are calculated to penetrate more deeply for longer wavelength excitation with respect to shorter wavelengths, and the penetration depth nearly doubles in comparison to that of the widely used silver thin films. As an application, indium thin films have been used for intrinsic protein chromophore (tryptophan) fluorescence detection. Nearly a 3.5-fold enhancement of intrinsic protein fluorescence along with a decreased decay time and increase in photostability were observed with SPCF. Subsequently, indium thin films can be utilized as a single assay platform in surface plasmon fluorescence spectroscopy (SPFS) over a broad spectral range and can potentially be a much better choice than other metallic thin films.