Plasmonic Field Effects on the Energy Transfer between Poly(p-phenyleneethynylene) Fluorescent Polymer and Au Nanocages

Colloidal gold nanocages (AuNCs) with a wall length of 60 nm and wall thickness of 6 nm were prepared by the galvanic replacement method. The AuNCs were assembled into monolayers with different percents of surface coverage, using the Langmuir-Blodgett technique. The same technique was used to coat the assembled monolayers of AuNCs with another monolayer of poly(p-phenyleneethynylene) (PPE) fluorescent polymer. The surface plasmon resonance (SPR) spectrum of the AuNCs red-shifts as the percent of AuNCs coverage increases due to growth of the interparticle surface plasmon resonance field coupling as interparticle distance decreases. While after coating the AuNCs array with PPE, the SPR of all the AuNCs monolayers arrays were red-shifted to the same position (693 nm). This unsystematic red-shift behavior is based on the presence of two plasmon fields (inside and outside of AuNCs) and is described by discrete dipole approximation (DDA) simulation. The polymer fluorescence intensity is found to decrease when the polymer is deposited over the AuNCs. This observation is due to the energy transfer between the excited PPE and the AuNCs as well interchain energy transfer. Upon turning on the plasmonic field (exciting the surface plasmon resonance band), it is found that the fluorescence intensity of the polymer increased to a maximum value at AuNCs percent coverage of 10% and then decreased as the interparticle separation distance decreased. DDA calculations showed that this is consistent with the dependence of the SPR field strength on the interparticle separation. Physically, this is a result of the interplay between the changes in the surface plasmon fields within the cavity of each nanoparticle and their external surface fields as a pair of AuNCs approach one another.