Effect of polyelectrolyte spacer thickness on fluorescence decay kinetics of IgG-FITC conjugates near plasmonic silver film

Enhancement of fluorescence near metallic nanostructures can significantly improve sensitivity of fluorescence-based detection methods and this effect has found wide applications in diagnostics and biosensors development. In the current paper we studied fluorescence decay kinetics of immunoglobulin G conjugates with fluorescein isothiocyanate near plasmonic silver film formed by colloidal nanoparticles having size of $\sim$40 nm. The emission kinetics was found to depend on thickness of a spacer composed from cationic/anionic polyelectrolytes (number of layers changed from 1 to 7), that separated fluorescent conjugate from the plasmonic film. Multiexponential fluorescence decay kinetics was modeled by continuous lifetime distribution of emitting centers using maximum entropy method. Based on theoretical calculations of radiative transition rate of fluorescent molecules near metal colloids we estimated ensemble-averaged fluorescence quantum yields of conjugates, located at different distances to plasmonic film, and found that the fluorescence enhancement for the conjugates near colloid nanoparticles was determined by changes in excitation efficiency but not due to growth of the emission quantum yield.