Molecular adsorption and resonance coupling at the colloidal gold nanoparticle interface

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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. © 2013 American Chemical Society.

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Journal of Physical Chemistry C

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