Improved Monodispersity of Plasmonic Nanoantennas via Centrifugal Processing

Abstract

Noble metal nanoparticle clusters underlie a variety of plasmonic devices and measurements including surface-enhanced Raman spectroscopy (SERS). Because of the strong dependence of plasmonic properties on nanoparticle cluster aggregation state, the elimination of non-SERS-active structures and the refinement of the nanoparticle cluster population are critical to realizing uniform and reproducible structures for plasmonic nanoantenna applications such as SERSbased sensors. In this Letter, we report a centrifugal sorting technique for gold core/ silica shell nanoparticles that host SERS reporter molecules at the gold/silica interface. The relatively massive nanoparticle clusters are sorted by sedimentation coefficient via centrifugation in a high-viscosity density gradient medium, iodixanol, which yields solutions that contain a preponderance of one aggregation state and a diminished monomer population, as determined by transmission electron microscopy, extinction spectroscopy, and SERS. A quantitative analysis of the nanoparticle sedimentation coefficients is presented, thus allowing this approach to be predictably generalized to other nanoparticle systems. SECTION Nanoparticles and NanostructuresT he intense electromagnetic field arising at the surface of metallic nanostructures from the excitation of the localized surface plasmon resonance (LSPR) allows for the enhancement of the Raman intensity of adsorbed molecules by a factor up to 4 Â 10 8 or greater. 1 The structures supporting this plasmonic phenomenon, known as surfaceenhanced Raman scattering (SERS), are diverse. The most sensitive examples, with an enhancement factor large enough to observe spectra at the single molecule level, 2-5 are aggregated metallic nanoparticles. Correlative structure-activity studies have indeed shown that the presence of a nanometersized junction 6,7 or crevice 1,8 creates the electromagnetic "hot-spot" (i.e., "nanoantenna") required to observe singlemolecule SERS. Recent investigations of the hot-spots at the junction of silver cubes 9 and gold pyramidal shells 10 or at the interface between a gold nanostar and a gold surface 11 have highlighted how the control over the structure of this nanometer-scale region is crucial for achieving high enhancement factors. Whereas the early fundamental studies of single-molecule SERS have been performed on inhomogeneous samples, the integration of plasmonic nanoantennas into reliable technological applications, such as high sensitivity biological and chemical sensors, requires improved structural reproducibility.Homogeneous nanostructure populations can be realized via precisely controlled fabrication or postsynthetic sorting techniques. Although much effort has been devoted to the controlled synthesis of nanoparticles, structural polydispersity remains an issue. 12-14 Consequently, postfabrication separation methods have become important for characterizing or refining populations of nanoparticles based on their size, shape, and aggregation state. 15 For example, electrophoretic methods, 1...

Solution‐processable exfoliated zeolite nanosheets purified by density gradient centrifugation

“…FeCo nanocrystals coated with graphitic shells were separated by nonlinear density gradient . Gold core/silica shell nanoparticles for application in plasmonics nanoantennas were purified using high viscosity iodixanol density gradient . Monodisperse Si nanocrystals were obtained by fractionation in density gradient of 40% 2‐4‐6 tribromotoluene in cholorobenzene …”

“…Barring a few, most of the DGC experiments have used aqueous density gradient (iodixanol, sucrose, etc.) for fractionation of particles.…”

Surface-enhanced Raman scattering-active Au/TiO2 films prepared by electrochemical and photochemical methods

“…The prepared SERS-active substrates demonstrated large Raman scattering enhancement for R6G with a detection limit of 2 Â 10 À12 M and an enhancement factor of 5.8 Â 10 7 [7]. As shown in the literature [8], Van Duyne groups reported a centrifugal sorting technique for Au core/SiO 2 shell NPs that host SERS reporter molecules at the Au/SiO 2 interface. The relatively massive nanoparticle clusters were sorted by sedimentation coefficient via centrifugation in a high-viscosity density gradient medium of iodixanol.…”

Centrifugal Shape Sorting and Optical Response of Polyhedral Gold Nanoparticles

“…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 al19 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%.…”

“…In particular, a highly concentrated, as‐synthesized gold nanoparticle (AuNP) solution ( Figure A) was loaded onto the density gradient medium iodixanol, which was selected because its high viscosity slows the sedimentation of AuNPs to the point where subtle differences can be exploited for sorting. In this case, the difference in size between the RD and BP AuNPs leads to the expectation that the BPs should sediment more quickly than RD, although other factors such as nanoparticle shape and surfactant loading could also play a role 17, 18. In particular, the sedimentation coefficient is given by S = m p (1 − ρ m / ρ p )/ f , where m p is the mass of the particle, ρ p the density of the particle, ρ m the density of the medium, and f the frictional coefficient.…”