Effect of Molecular Position and Orientation on Adsorbate-Induced Shifts of Plasmon Resonances

Localized surface plasmon resonances (LSPRs) of metallic nanoparticles are affected by their surroundings and in particular by the presence of adsorbed molecules on their surface. This effect is central to their application in LSPR sensing. We here investigate how the adsorbed molecule orientation on the surface and position, notably with respect to an electromagnetic hot spot, affect the amplitude of the resonance shift, and therefore the LSPR sensing sensitivity. We use a recently developed effective anisotropic dielectric function describing a homogeneous shell of adsorbed molecules combined with anisotropic Mie theory calculations for core-shell spherical systems or finite-element modeling for nonspherical or partial shell configurations. We show that the induced plasmon resonance shift per molecule is strongly correlated with the near-field enhancement experienced by the adsorbed molecules. Molecules with their main optical axis perpendicular to the surface and those located at hot spots can therefore induce a much larger resonance shift, by at least 1 order of magnitude. This work suggests that large improvements in LSPR sensing sensitivity could be achieved with new schemes including targeted adsorption at hot spots with carefully engineered molecular orientation.