Assessing Interparticle J-Aggregation of Two Different Cyanine Dyes with Gold Nanoparticles and Their Spectroscopic Characteristics

While nanopartide plasmonic coupling is known to be responsible for the enhancement of the local electrical field that determines optical or spectroscopic properties, understanding of the structural details of the interparticle interactions remains elusive. This report describes findings of an investigation of plasmonic coupling of gold nanoparticles via J-aggregation of cyanine dyes to define the interparticle interaction. The adsorption of two cyanine dye molecules with subtle differences in structures on gold nanoparticles of different sizes and their resulting interparticle pi-pi interactions, or J-aggregation, were examined to assess the interparticle-interaction-induced changes of spectroscopic properties, surface plasmon resonance absorption, and surface-enhanced Raman scattering. The results demonstrate that these two spectroscopic properties work in concert with plasmonic coupling in the kinetic process, as evidenced by the comparable apparent rate constants determined in terms of nanoparticle dimerization or aggregative growth in the solution. This finding is further substantiated by examining the effects of the dye structure, the particle size, and the solution pH on the spectroscopic characteristics. The experimental data are supported by theoretical simulation of the spectroscopic properties in terms of the plasmonic resonance absorption and the electrical field enhancement in terms of nanopartide dimer models, which have implications in the better design of interparticle structures to harness the nanoscale interparticle molecular pi-pi interactions for a wide range of technological applications.