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Collective Plasmonic Resonances on Arrays of Cysteine-Functionalized Silver Nanoparticle Aggregates

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posted on 07.08.2014, 00:00 by Mária Csete, Anikó Szalai, Edit Csapó, László Tóth, Anikó Somogyi, Imre Dékány
UV–visible spectroscopy on biofunctionalized spherical silver nanoparticle containing dispersions has shown that bioconjugation with L-cysteine results in aggregate formation, which causes appearance of UV and red-shifted absorptance maxima simultaneously. On the basis of comparative study of spectra registered experimentally and computed numerically, the simplest linear and wavy aggregate array geometries resulting in splitting according to the measurements were determined. Inspection of the near-field distribution has shown that collective plasmonic resonances develop both on linear and on wavy aggregates, which are influenced by the γ azimuthal orientation qualifying the E-field oscillation direction and by the φ incidence angle during p-polarized light illumination. These resonances originate from hybrid near- and far-field coupling phenomena, of which relative dominance is determined by the aggregate-arrays’ geometry and by the illumination direction. At perpendicular incidence the collective resonances manifest themselves dominantly in even and odd eigenmodes at the UV and red-shifted maxima, respectively. The eigenmodes’ excitation efficiency sensitively depends on the E-field projection onto the aggregates’ long axes, when the azimuthal angle is varied. In addition to resonant eigenmodes developing at analogous azimuthal and polar angles, squeezed guided plasmonic and standing resonator modes accompanied by enhanced EM-field appear at oblique incidence, due to grating coupling on the aggregate arrays.