Molecular
Adsorption and Resonance Coupling at the
Colloidal Gold Nanoparticle Interface
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Abstract
Second
harmonic generation is used to investigate the adsorption
properties of malachite green, brilliant green, and methyl green to
the surface of 80 nm colloidal gold nanoparticles capped with mercaptosuccinic
acid in water. The experimental results are fit using the modified
Langmuir model to obtain the free energies of adsorption and the adsorbate
site densities for each cationic triphenylmethane molecular dye. Malachite
green is observed to bind more strongly than brilliant green or methyl
green to the nanoparticle surface but has a lower adsorbate site density,
indicating differences in image-charge effects, adsorbate–adsorbate
repulsions, and adsorption tilt angles. Complementary measurements
from extinction spectroscopy show plasmonic and molecular resonance
coupling leading to the formation of new polaritonic states and Fano-type
resonances with corresponding plasmon and molecular spectral depletions
as the adsorbate concentration is increased. The changes in the resonance
coupling spectra are compared to the second harmonic generation molecular
adsorption results and demonstrate the sensitivity of plasmonic–molecular
interactions