Steven L. Tripp

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.

Microcantilever mechanics in flowing viscous fluids

Microcantilevers are often deployed in flowing fluids to measure local flow velocities or to detect rapidly the nanomechanical binding of trace quantities of target analytes. The authors investigate the flow-induced mechanics of microcantilevers by deriving a semianalytical theoretical model for the nanoscale deflections of an elastic microcantilever due to a laminar viscous flow incident upon it. Conversely, the model allows for the estimation of the local flow velocities based on measured microcantilever deflection. Careful experiments performed on silicon microcantilevers in flowing nitrogen confirm the theoretical predictions up to a critical flow rate, beyond which unsteady flow-induced vibrations are seen to occur.

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.

Calixarene-stabilised cobalt nanoparticle rings: Self-assembly and collective magnetic properties

Calixarenes can be used to promote the self-assembly of thermoremanent cobalt nanoparticles into bracelet-like rings below 100 nm in diameter. These kinetically stable assemblies are regulated by the equilibrium between enthalpic gain (dipole–dipole and long-range van der Waals interactions) and entropic loss, analogous to the thermodynamic balance of forces governing supramolecular self-assembly. Examination of the Co nanoparticle rings by electron holography (an electron microscopy technique for imaging in-plane magnetic induction) reveals the existence of chiral flux closure (FC) domains at room temperature, comprising a ‘racemic’ mixture of clockwise and anticlockwise states. Furthermore, these FC polarisations can be reversed by applying out-of-plane magnetic pulses (H z ) in alternating directions. This switching behaviour has no known analogy at the macroscopic level, and may represent a uniquely nanoscale phenomenon.

Off-Axis Electron Holography of Self-Assembled Co Nanoparticle Rings

We use off-axis electron holography in the transmission electron microscope (TEM) to study magnetic flux closure (FC) states in self-assembled nanoparticle rings that each contain between five and eleven 25-nm-diameter Co crystals. Electron holograms are acquired at room temperature in zero-field conditions after applying chosen magnetic fields to the samples in situ in the TEM by partially exciting the conventional microscope objective lens. Mean inner potential contributions to the phase shift are determined by turning the samples over, and subsequently subtracted from each recorded phase image to obtain magnetic induction maps. Our results show that most nanoparticle rings form FC remanent magnetic states, and occasionally onion-like states. Although the chiralities (the directions of magnetization) of the FC states are determined by the shapes, sizes and positions of the constituent nanoparticles, reproducible magnetization reversal of each ring can be achieved by using an out-of-plane magnetic field of between 1600 and 2500 Oe.

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.