Nonlinear and ultrafast spectroscopy of molecular dye interactions with colloidal plasmonic nanoparticles

Mol. dye interactions with colloidal gold and gold- silver- gold core- shell- shell nanoparticles are studied using second harmonic generation (SHG) , extinction spectroscopy, and transient absorption spectroscopy. The adsorption isotherms of several dyes such as malachite green, brilliant green, and rhodamine 110 to the colloidal nanoparticle surface in water are measured with SHG and the results are fit using the modified Langmuir model to det. the free energies of adsorption and the adsorbate site densities. Complementary measurements of the extinction spectra of the combined dye and nanoparticle solns. with subtractions from the spectra of the dye and nanoparticles alone at corresponding concns. reveal strong polaritonic states from resonant coupling that depend on the dye- plasmonic nanoparticle interactions. The resonant coupling spectroscopy agrees with computational simulations using a multiscale hybrid /classical approach, showing polariton peaks that overlap with a Fano- type profile. The plasmonic spectra of core- shell- shell nanoparticles are controlled by changing the shell thicknesses for improved spectral overlap with the adsorbed dyes, resulting in significantly enhanced resonant coupling peaks. Addnl., transient absorption spectroscopy on excited- state dynamics of the dye- nanoparticle solns. show the convolution of plasmonic and mol. dynamics to study effects from energy transfer, mol. hindrance, and optical field enhancements. The results are compared to fluorescent quenching and enhancement measurements to understand the overall, time- dependent optical and energetic interactions between dye mols. and colloidal plasmonic nanoparticle surfaces. These linear, nonlinear, and ultrafast spectroscopic investigations provide important information that can be utilized for improved plasmon- enhanced mol. sensing applications in aq. soln