Surface-enhanced Raman scattering on tunable plasmonic nanoparticle substrates

Abstract

Au and Ag nanoshells are investigated as substrates for surfaceenhanced Raman scattering (SERS). We find that SERS enhancements on nanoshell films are dramatically different from those observed on colloidal aggregates, specifically that the Raman enhancement follows the plasmon resonance of the individual nanoparticles. Comparative finite difference time domain calculations of fields at the surface of smooth and roughened nanoshells reveal that surface roughness contributes only slightly to the total enhancement. SERS enhancements as large as 2.5 ؋ 10 10 on Ag nanoshell films for the nonresonant molecule p-mercaptoaniline are measured.nanoparticles ͉ nanoshells ͉ plasmons ͉ spectroscopy S ince the initial discovery of surface-enhanced Raman scattering (SERS) (1-4), understanding how the local electromagnetic environment enhances the substrate-adsorbate complex's spectral response has been of central importance. It has become increasingly evident that plasmon resonances of the metallic substrate provide intense, local optical-frequency fields responsible for the electromagnetic contribution to SERS (5-7).The lack of reliable techniques for controlling the properties of the local field at the metal surface has been a major experimental limitation in the quantification and understanding of SERS. A striking example of this is the series of experiments reporting enormous SERS enhancements of 10 12 to 10 15 for dye molecules adsorbed on surfaces of aggregated Au and Ag colloid films (6,8,9). The SERS enhancements reported in these experiments have been attributed to localized plasmons, or ''hot spots,'' occurring randomly across this film that fortuitously provide the appropriate electromagnetic nanoenvironment for large SERS enhancements (10). More recent studies have shown that localized plasmons giving rise to very large field enhancements can be formed at the junctions between adjacent nanoparticles (11,12). These plasmons can be described within the plasmon hybridization picture as dimer resonances (13-15). Likewise, self-similar geometries also provide a means for developing large field enhancements (10, 16).Several experimentally realizable geometries, such as triangles (17), nanorings (18), and nanoshells (19), support well defined plasmon resonances whose frequencies can be controlled by judicious modification of the geometry of the nanoparticle. Each of these nanostructured geometries offers its own unique nearfield properties: plasmon resonant frequency, spatial distribution of the near-field amplitude across the surface of the nanostructure, orientation dependence on polarization of the incident light wave, and spatial extent of the near field. The near-field properties of metallic nanoparticles can be calculated very precisely by a variety of methods, such as analytic Mie scattering theory for high-symmetry geometries, and numerical methods such as the discrete dipole approximation (DDA) (20) and the finite difference time domain (21) methods for nanoscale objects of reduced symmetry. Thus, we can approac...

Application of Surface-Enhanced Raman Spectroscopy for Detection of Beta Amyloid Using Nanoshells

“…Additionally, by controlling the geometry of the nanoshells, the SERS enhancement for a layer of nonresonant molecules bound to the surface of the nanoshells can be controlled with quantitative agreement between theoretical and experimental results [48]. Solutions of nanoshells are limited by significant reabsorption of the backscattered SERS signal by other nanoshells, which limits the observed SERS enhancement [49]. Thus, it is desired that the nanoshells be deposited onto a substrate to allow for larger SERS enhancements due to the simplified collection geometry.…”

“…Nanoshells are spherical nanoparticles with a dielectric core surrounded by a metal shell with plasmon resonances that can be adjusted by altering the ratio of the core diameter to the shell diameter [47]. Nanoshells have been previously shown to provide significant SERS enhancements of 10 6 -10 10 in the near IR by tuning the plasmon resonance to the excitation laser wavelength [48][49][50]. Additionally, by controlling the geometry of the nanoshells, the SERS enhancement for a layer of nonresonant molecules bound to the surface of the nanoshells can be controlled with quantitative agreement between theoretical and experimental results [48].…”

Engineering the synthesis of silica–gold nano-urchin particles using continuous synthesis

“…5,6 Gold nanoparticles have been synthesized by a variety of approaches for diverse applications in catalysis, bio-imaging, drug delivery and photovoltaics. Their surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS) properties have been of particular interest for photothermal therapy and molecular sensing, respectively [7][8][9][10][11][12][13][14][15] . SERS active structures have been fabricated by complex with top-down methods, 4,12,16,17 but wet-chemical synthesis of gold nanoparticles offer potential advantages in simple, high yield, and limited scalable synthesis.…”

“…12,18,19,20,21,22 Gold nanostructures, such as nanorods, nanowires, tetrapods, nanoplates and star-shaped particles are particularly attractive for many applications because of their strong confinement of the electromagnetic field and high enhancement that can be tuned over a wide range of optical wave lengths in comparison with ordinary spherical structures. 11,[23][24][25][26][27] Combination of the metal nanostructure with a support material, such as in core-shells, raspberries, and crescents, affords additional advantages in tuning optical properties, improved chemical/mechanical stability, and ease of handling compared to free-standing nanoparticles.Batch methods, 20 specialized ligands 28 and additional electrochemical treatments 29 have typically been needed to synthesize these optical architectures, but create challenges in achieving desired optical properties and sufficient reproducibility. Herein we develop a method based on molecular self-assembly and reduction chemistry of gold species to grow gold nanobranches on the surface of silica nanoparticles.…”

Ultrasensitive surface-enhanced Raman scattering detection in common fluids

“…Among various analytical techniques, surface-enhanced Raman scattering (SERS) is among the most promising methods in detecting trace amounts of molecules owing to its high molecular specificity (i.e., differentiation between different types of molecules) and high sensitivity (i.e., the lowest analyte concentration from which SERS signals are distinguishable from the noise signal of a control sample) (9)(10)(11)(12)(13)(14)(15)(16)(17). Extensive studies have focused on the structural optimization of the SERS substrate to improve SERS sensitivity (18)(19)(20)(21)(22), but there are two important roadblocks that limit its practical applications. First, SERS detection in liquid media relies on highly statistical binding of analytes to the SERS-sensitive regions (or "hot spots"), a consequence of the diffusive nature of the analytes (23)(24)(25)(26)(27).…”