Plasmon-Enhanced Emission of a Phthalocyanine in Polyelectrolyte Films Induced by Gold Nanoparticles

We report on the plasmon-enhanced fluorescence emission achieved in an organic/inorganic hybrid multilayered poly electrolyte film incorporating aluminum tetrasulfonated phthalocyanine (AlPcTS) and gold nanoparticles (AuNPs). The induced effect can be tuned by the number of polyelectrolyte layers separating AlPcTS and AuNPs, reaching the most favorable conditions with seven polyelectrolyte (PE) layers. The increase in the emission intensity of AlPcTS was measured by steady-state emission as well as by confocal fluorescence microscopy. The films are optically heterogeneous, and plasmon-enhanced emission is stronger in spatially confined "hot spots" that are observed in the fluorescence intensity images. Concurrent decrease in the average fluorescence lifetime was revealed by fluorescence lifetime imaging microscopy (FLIM). The trend in the average lifetime inferred from FLIM images is confirmed by the lifetime distributions, which shift to shorter decay times when the number of PE layers is increased from 3 to 7 PE layers. This trend reverses with 9 and 11 PE layers. A distribution of plasmon resonance effects due to interparticle interactions, as well as the film's heterogeneity, is advanced to explain the photophysical behavior of phthalocyanines positioned at variable distances and heights in the gaps between AuNPs.