Johnson and Hupp
Fig. 3. Comparison of Rayleigh (open circles) and hyper-Rayleigh (filled circles) signals from 13-nm gold nanoparticles after aggregation induced by addition of 6.0 M NaCl solution (15).
collective symmetry (centrosymmetry) to permit the observation of an additional NLO response that would otherwise be cancelled. Furthermore, the reduction in chromophoric centrosymmetry upon small aggregate formation is sufficiently great that further aggregation, and further symmetry reduction, offer no additional advantage. Although shifts in plasmon band energy and intensity appear not to play a dominant role in the studies of salt-induced aggregation summarized in Figs. 2 and 3, these effects might be important under other conditions (other excitation wavelengths, other aggregation agents, etc.) The formation of ellipsoidal aggregates rather than highly asymmetric aggregates has been observed for pyridineinduced aggregation (24). Note that a perfect ellipsoid, like a perfect sphere, possesses an inversion center and therefore should offer no advantage in a nonlinear scattering experiment. In any case, gold nanoparticle aggregates of approximately spheroidal shape display a substantially enhanced HRS response (23). They also display a broad extinction in the near IR not seen for the component spherical particles; this feature has been assigned as a longitudinal plasmon absorbance attributable to an electromagnetic response of each of the aggregates as a whole. (The precise shape and location of the longitudinal band depend on aggregate size and
Chemistry of metal nanoparticles