Colm T. Mallon

The Effect of Ag Nanoparticles on Surface-Enhanced Luminescence from Au Nanovoid Arrays

Studies comparing the effect of adding two different nanoparticle compositions on the plasmonic properties of Au nanovoid arrays were undertaken. Surface-enhanced resonance luminescence and surface-enhanced resonance Raman studies comparing dispersed Ag nanoparticles and Ag nanoparticle aggregates on gold nanovoid arrays were undertaken. These studies showed that using Ag nanoparticle aggregates increased both luminescence and Raman efficiency relative to when dispersed nanoparticles were used; in addition, these studies also showed that adding dispersed Ag nanoparticles supported a more reproducible enhancement in luminescence and Raman across the substrate compared to using Ag nanoparticle aggregates. Finite element analysis simulations indicated that surface plasmon polariton distribution in the sample was affected by the presence of the Ag nanoparticles on the Au nanovoid array.

Electrochemical desorption of fibrinogen from gold.

The electrochemically induced desorption of Oregon green labeled fibrinogen layers from clean gold surfaces at negative potentials has been probed using capacitance, fluorescence microscopy, and atomic force microscopy. Capacitance measurements on fibrinogen layers indicate that desorption occurs at potentials more negative than -0.8 V and that complete desorption occurs when the electrode is biased at -1.2 V. Significantly, the fluorescence intensity initially increases as the dye labeled protein is electrochemically desorbed due to a decrease in quenching by the gold surface. Following this initial increase, the protein diffuses into solution and the fluorescence intensity decreases over time. More than 90% of the dye labeled fibrinogen is desorbed and diffuses out of the confocal volume in less than 2000 s when the potential is stepped to -1.2 V. AFM before and after application of the desorbing potential confirms removal of the protein. Collection of the desorbed protein in solution reveals a surface coverage of (4.0 +/- 2.3) x 10(-13) mol cm(-2) or an area of occupation of 400 +/- 140 nm(2) per molecule, which indicates that the protein is not extensively spread on the bare gold surface. Significantly, SDS-PAGE analysis indicates that the adsorption-desorption cycle dramatically effects the protein structure, with the electrochemically desorbed fibrinogen showing extensive fragmentation compared to native protein.

Fabrication of gold sphere to cuboid nanoarrays using PDMS templates

A novel polymer based templated approach to creating highly ordered gold arrays with either positive or negative nano-scale features of controlled shape from truncated spheres to cuboids is presented.

Single nanocavity electrodes: fabrication, electrochemical and photonic properties

Nanosphere lithography has been used to create spherical recessed electrodes of nanometre dimensions capable of enhancing fluorescent signals.

Mercury-Platinum Tunneling Junctions Incorporating Supramolecular Host-Guest Assemblies

Molecular junctions incorporating monolayers of surface-active cyclodextrins have been formed by bringing macroscopic platinum and mercury electrodes together. Monolayers of the host di-6A,6B-deoxy-6-(4-pyridylmethyl)amino-gamma-cyclodextrin (CD) have been formed on platinum electrodes and backfilled with 1-nonanethiol. Significantly, the tunneling current for bilayer junctions in which the mercury surface is modified with a C16 alkane thiol depends markedly on the nature of the CD guest. Junctions where nonconjugated guests, such as adamantane, are included in the CD show an order of magnitude lower current than junctions incorporating the conjugated guest C60. Moreover, monolayer junctions of CD backfilled with 1-nonanethiol exhibit potential-dependent currents in the presence of CD guest molecule [Co(biptpy)2](2+) but not for [Co(tpy)2](2+), which is structurally analogous but cannot associate with CD. Significantly, unlike bilayer systems, when only one electrode is modified, mechanical pressure is required to form a stable tunneling junction and these junctions cannot be formed and separated reversibly.

Reflectance properties of gold nano-cavity spherical and cuboid molded arrays

Gold nano-cavity arrays supported on polydimethylsiloxane (PDMS) have been created using colloidal lithography. PDMS is cured on top of hexagonally close packed arrays of polystyrene spheres of diameter 820 nm resulting in a close packed sphere imprinted polymer block. The depth of the imprints is 200 nm, indicating the whole sphere is not entrapped in the polymer during curing. The spherical nature of the imprint can be deformed by stretching of the flexible polymer, thus creating cuboid shaped arrays. Finally, the arrays are coated with a 100 nm gold layer, which conforms to the polymer surface to create either spherical or cuboid shaped gold nano-cavities. Experiments show that the reflectance properties of the arrays are critically dependent on the shape of the cavity. Spherical shaped cavity arrays display diffuse reflectance peaks at wavelengths slightly shorter than the diameter of the templating sphere, which are absent in the cuboid arrays. Both spherical and cuboid arrays show reflectance which is strongly dependent on the angle of incidence, with the cuboid arrays showing differing spectra depending on the direction of the impinging light with relation to the axis of stretching. The changes in optical behavior between the spherical and cuboid cavity arrays is discussed with relation to the change of shape of the patterning feature at the interface.

Detecting Disease Biomarkers Using Nanocavities and Nanoparticle Composites

The convergence of electrochemistry, materials, photonics and biomedical science at the nanoscale opens up significant opportunities for developing advanced sensors. In this contribution, we present examples of our use of nanometer dimensioned electrodes, nanocavities and nanoparticle-metallopolymer composites to create high sensitivity detection platforms and materials for detecting proteins and nucleic acids. The application of these approaches in the diagnosis and prognosis of cancers such as neuroblastoma, as well as point-of-care detection of infectious disease, will be discussed.