Centrifugal Shape Sorting and Optical Response of Polyhedral Gold Nanoparticles (original) (raw)

Metallic nanostructures are of high interest because their optical properties can be tuned throughout the visible and nearinfrared portions of the electromagnetic spectrum by adjusting nanoparticle shape, size, composition, and local dielectric environment. [ 1-4 ] In particular, the localized surface plasmon resonance (LSPR) properties of metal nanoparticles have shown promising applications in chemical and biological sensing, [ 5-7 ] waveguiding, [ 8 ] single-particle tracking, [ 9 ] and surface enhanced Raman spectroscopy (SERS). [ 10 , 11 ] Since inhomogeneous broadening resulting from nanoparticle size and shape variations causes an increase in the linewidth of bulk LSPR spectra, monodispersity of the nanoparticle population is critical for optimal performance in LSPR-based devices and sensors. [ 12 ] Towards that end, previous studies have focused on obtaining more homogeneous nanoparticle populations by means of post-synthetic separation techniques, including size exclusion chromatography, [ 13 ] gel electrophoresis, [ 14 ] and centrifugation. [ 15 ] Among these methods, density gradient centrifugation has proven to be particularly successful, resulting in narrow distributions of gold nanoparticle diameters, [ 16 ] shapes, [ 17 ] and aggregation state. [ 18 ] Recently, Akbulut et al. [ 19 ] reported the use of aqueous multiphase systems as media for rate-zonal centrifugation to separate the reaction products (i.e., nanorods, nanospheres, and large particles) of a nominal gold nanorod synthesis, ultimately increasing the nanorod purity from 48% to 99%. The shape-dependent optical properties of metal nanostructures have also motivated efforts to realize new nanoparticle shapes via novel synthetic routes [ 20-29 ] and to correlate nanoparticle structure with plasmonic behavior. [ 21 , 30-33 ] Among available nanoparticle shapes, gold bipyramids (BPs) are of interest because of their sharp tips, which lead to strong localized fi eld enhancement and high sensitivity to the surrounding dielectric www.advmat.de www.MaterialsViews.com