Pyramids: a platform for designing multifunctional plasmonic particles.

This Account explores nanofabricated pyramids, a new class of nanoparticles with tunable optical properties at visible and near-infrared wavelengths. This system is ideally suited for designing multifunctional plasmonic materials for use in diagnostics, imaging, sensing, and therapeutics. The nanofabrication scheme that we developed (called PEEL) for these asymmetric metal particles is extremely versatile and offers several advantages over synthetic methodologies. The PEEL approach yields pyramids with variable sizes, thicknesses, and multimetal compositions, as well as blunt or ultrasharp tips or no tips. In addition, we have prepared pyramids with site-specific chemical and biological functionality on different portions of the pyramids. This is an important design feature for biological applications, as suggested by the generation of amphiphilic gold pyramids functionalized with alkanethiols on the hydrophobic portions and DNA on the hydrophilic portions. The optical characteristics of these pyramids depend on particle orientation, wavevector direction, and polarization direction and can be tuned. Using the multipolar surface plasmon resonances of large (>250 nm) pyramids, imaging and spectral identification of pyramid orientation in condensed media was possible. We were also able to direct pyramids to assemble into one- and two-dimensional arrays with interesting optical properties. Furthermore, modification of the PEEL fabrication scheme allowed the production of multimaterial pyramidal structures with complex attributes, highlighting the power of this platform for exacting nanometer-scale control over particle structure and composition.