Numerical investigation of the plasmonic properties of bare and cysteine-functionalized silver nanoparticles

The absorption spectra of different aqueous dispersions containing silver nanoparticles were computed by finite element method and compared to spectra determined by UV-Visible spectroscopy. This comparative study proved that the spectrum measured on the aqueous dispersion of bare silver nanoparticles with absorptance maximum at lambda(meas) = 391 nm corresponds to the characteristic UV surface plasmon band of spherical nanoparticles with 8.25 nm diameter. The presence of aggregates in aqueous dispersions resulted in splitting on the spectrum, when the surface of the silver nanoparticles with c(Ag) = 2x10(-4) M concentration was functionalized by L-cysteine with c(Cys)=5.7x10(-6) M concentration. The simplest aggregate-geometries that exhibit resonance at the measured absorptance maxima were determined by varying the number of silver nanoparticles, and the inter-particle distance in linear chains, and taking 0.45 nm thick cysteine-shell into account. The FEM computations proved that the primary maxima in the UV involve quadrupolar modes, while the secondary maxima red-shifted to lambda(meas_2)=567 nm at pH=2.98 and lambda(meas_2)'=588 nm at pH=4.92 originate from coupled dipolar plasmon resonances on extended aggregates aligned along the E-field oscillation direction. The non-aggregated particles and the aggregates rotated with respect to the E-field oscillation direction significantly contribute to the UV peak.