Surface Plasmon Coupled Fluorescence in the Visible to Near-Infrared Spectral Regions using Thin Nickel Films: Application to Whole Blood Assays

Abstract

Nickel thin films thermally evaporated onto glass supports are used to demonstrate surface plasmon coupled fluorescence (SPCF) over a broad 400 nm wavelength range (400-800 nm) for potential assays that can be run in buffer and/or whole blood. In contrast to traditional fluorescence-based assays, SPCF converts otherwise isotropic emission into highly directional and polarized emission, an attractive concept for surface assays. Theoretical Fresnel calculations performed in the ultraviolet to near-infrared spectral range (344-1240 nm) predict the near-field generation of surface plasmons in 15 and 20 nm nickel thin films. The angles of minimum reflectivity for nickel thin films with 10 nm SiO(x) and 30 nm polymer overcoats over the 428-827 nm wavelength range occur over a 10 degree range. To experimentally corroborate the theoretical calculations, a polymeric solution of five different fluorophores, POPOP (lambda(max, emission) = 428 nm), FITC (517 nm), S101 (600 nm), Zn PhCy (710 nm), and IR 780 (814 nm), were spin coated separately onto 15 and 20 nm nickel thin films. SPCF intensity (s- and p-polarized) from fluorophores at the corresponding emission lambda(max) was measured at angles between 0-90 degrees. In addition, the free-space emission and SPCF intensity of FITC on 20 nm nickel thin film were also measured to demonstrate the angular-dependent nature of SPCF. SPCF from nickel thin films was p-polarized and highly directional with lambda(max) confirmed at an angle of 65 degrees for all the fluorophores as predicted by Fresnel calculations. The utility of nickel thin films for whole blood bioassays is demonstrated with a long-wavelength fluorophore, where the SPCF intensity of Zn PhCy (50 pM-50 microM) in whole blood at 710 nm was measured at 65 degrees. Using Fresnel calculations it is also predicted that the evanescent field above the nickel films penetrates deeper into solution than for other metals used to date for SPCF, an attractive notion in SPCF-based biosensing applications.