Partial Decoupling in Aggregates of Silanized Gold Nanorods

The introduction of gold nanorods as contrast agents for applications in biomedical optics is receiving significant interest. However, particle aggregation in endocytic vesicles may jeopardize their great potential of photothermal or photoacoustic conversion because the multitude of couplings in tight clusters of plasmonic particles deteriorates their functional fingerprints of optical extinction. Here, we investigate the benefit of silanization on self-aggregated gold nanorods as a function of their shell width. We have increased this parameter from zero up to a value similar to the particle diameters, that is, about 12 nm in our case. With this shell width, we have recovered two distinct peaks in the green and near-infrared sides of the experimental spectrum, with the latter about 2-fold higher than its uncapped counterpart. This result points to a significant decoupling between neighboring particles. We interpret our experimental results in terms of the residual couplings in building blocks that yield the greatest optical response, which are pairs of particles in a so-called end-to-end configuration. Our simulations account for the coexistence of particles with different shapes and orientations and reproduce well both their longitudinal and transverse modes of plasmonic oscillations.