Beomseok Kim

Tunable Surface-Enhanced Raman Scattering from Large Gold Nanoparticle Arrays

Raman signal enhancements in excess of 10(7) can be achieved at near-infrared wavelengths when mid-nanometer sized gold particles self-organize into close-packed planar arrays. These substrates generate stable surface-enhanced Raman scattering which changes dramatically as a function of periodic structure and excitation wavelength.

Self-assembly of Resorcinarene-stabilized Gold Nanoparticles: Influence of the Macrocyclic Headgroup

Gold nanoparticles can be encapsulated by various resorcinarene derivatives and assembled into monolayer films at solvent interfaces. Surface charge plays a critical role in both nanoparticle extraction and self-assembly: the degree of monolayer formation and local two-dimensional (2D) order within the nanoparticle arrays is dependent on the chemical nature of the resorcinarene headgroup as well as the presence of other electrolytes. Cluster size distribution analysis can be used to parameterize local 2D order within the arrays in a quantitative manner, based on mean cluster sizes and fractional hexagonal close-packed (hcp) cluster formation. 2D nanoparticle arrays can also be prepared in some cases using Langmuir–Blodgett techniques. These studies demonstrate that resorcinarenes with chemisorptive headgroups promote the self-assembly of well-ordered 2D arrays.

Extraction and Dispersion of Large Gold Nanoparticles in Nonpolar Solvents

Gold nanoparticles up to 70 nm in diameter could be extracted from aqueous solutions into nonpolar organic solvents by tetrathiolated resorcinarenes 1 and 2. The resorcinarene-coated nanoparticles formed stable dispersions in toluene and chloroform and could be passed through a crosslinked polystyrene column without significant degradation, but exhibited variable resistance to alkanethiol-induced flocculation. Gold nanoparticles encapsulated by resorcinarene 2 were found to be exceptionally stable even in the presence of propanethiol and dodecanethiol, with an approximate dispersion half-life of one month at room temperature.

Tuning the Optical Properties of Large Gold Nanoparticle Arrays

Gold nanoparticles in the mid-nanometer size regime can undergo self-organization into densely packed monoparticulate films at the air-water interface under appropriate passivation conditions. Films could be transferred onto hydrophilic Formvar-coated Cu grids by horizontal (LangmuirSchaefer) deposition or by vertical retraction of immersed substrates. The latter method produced monoparticulate films with variable extinction and reflectance properties. Transmission electron microscopy revealed hexagonally close-packed arrays on the micron length scale. The extinction bands of these arrays shifted by hundreds of nanometers to near-infrared wavelengths and broadened enormously with increasing periodicity. Large particle arrays also demonstrated extremely high surface-enhanced Raman scattering (SERS), with enhancement factors greater than 10 7 . Signal enhancements could be correlated with increasing periodicity and are in accord with earlier theoretical and experimental investigations involving nanoparticle aggregate structures.

Nanostructured materials as biomolecular sensors for cell transport

Surface-enhanced Raman scattering (SERS) is capable of analyte detection. Advancement of SERS as an analytical tool is dependent on the fabrication of substrates with stable and reproducibly high activities. Efforts in our laboratory are addressing these issues by developing new methods for creating well-defined nanostructured substrates from gold particles in the mid-nanometer size regime. These nanoparticle ensembles demonstrate size-tunable optical properties including stable and reproducible SERS. Very recently we have determined that the nanostructured materials are compatible with living cells, and are thus excellent candidates as online sensors of cell membrane transport activity.