Simultaneous excitation of propagating and localized surface plasmon resonance in nanoporous gold membranes

Materials multifunctionality for optical sensing of adsorbates has obvious advantages--in addition to the potential for greater sensitivity, the different length scales associated with a variety of optical phenomena allow a greater variety of adsorption characteristics to be examined. Here, we show that ultrathin (~100 nm) nanoporous gold membranes possess features of both planar metal films that exhibit propagating SPR excitations and nanofeatured metals that exhibit localized SPR excitations. This is the first report of such multifunctionality in an optically active metal. We give illustrative examples of using this material to probe biorecognition reactions and to probe the structure evolution of a layer-by-layer deposition of charged dendrimers. Our results are consistent with the very different lengths of the tail of the evanescent field decays associated with each of these plasmon excitation modes.