Alan O’Riordan

Single Nanoskived Nanowires for Electrochemical Applications

In this work, we fabricate gold nanowires with well controlled critical dimensions using a recently demonstrated facile approach termed nanoskiving. Nanowires are fabricated with lengths of several hundreds of micrometers and are easily electrically contacted using overlay electrodes. Following fabrication, nanowire device performance is assessed using both electrical and electrochemical characterization techniques. We observe low electrical resistances with typical linear Ohmic responses from fully packaged nanowire devices. Steady-state cyclic voltammograms in ferrocenemonocarboxylic acid demonstrate scan rate independence up to 1000 mV s(-1). Electrochemical responses are excellently described by classical Butler-Volmer kinetics, displaying a fast, heterogeneous electron transfer kinetics, k(0) = 2.27 ± 0.02 cm s(-1), α = 0.4 ± 0.01. Direct reduction of hydrogen peroxide is observed at nanowires across the 110 pM to 1 mM concentration range, without the need for chemical modification, demonstrating the potential of these devices for electrochemical applications.

Polarization tunable transmission through plasmonic arrays of elliptical nanopores

Polarization dependent transmission through thin gold films bearing arrays of elliptical nanopores and assembled at transparent substrates is explored. Far field transmission spectra with incident light polarized along the short and long axis of the ellipses show asymmetric peaks. Near-field finite difference time domain simulated electric field profiles suggest these features are related to Fano resonances between the (± 1, 0) Surface Plasmon Polariton mode and the ( ± 1, 0) Rayleigh Anomaly. The unique spectral signature of these samples makes them attractive for visible and near infrared tags for anti-counterfeiting applications.

Dielectrophoretic self-assembly of polarized light emitting poly(9,9-dioctylfluorene) nanofibre arrays

Conjugated polymer based 1D nanostructures are attractive building blocks for future opto-electronic nanoscale devices and systems. However, a critical challenge remains the lack of manipulation methods that enable controlled and reliable positioning and orientation of organic nanostructures in a fast, reliable and scalable manner. To address this challenge, we explore dielectrophoretic assembly of discrete poly(9,9-dioctylfluorene) nanofibres and demonstrate site selective assembly and orientation of these fibres. Nanofibre arrays were assembled preferentially at receptor electrode edges, being aligned parallel to the applied electric field with a high order parameter fit (∼ 0.9) and exhibiting an emission dichroic ratio of ∼ 4.0. As such, the dielectrophoretic method represents a fast, reliable and scalable self-assembly approach for manipulation of 1D organic nanostructures. The ability to fabricate nanofibre arrays in this manner could be potentially important for exploration and development of future nanoscale opto-electronic devices and systems.

High aspect ratio nano-fabrication of photonic crystal structures on glass wafers using chrome as hard mask

Wafer-scale nano-fabrication of silicon nitride (Si x N y ) photonic crystal (PhC) structures on glass (quartz) substrates is demonstrated using a thin (30 nm) chromium (Cr) layer as the hard mask for transferring the electron beam lithography (EBL) defined resist patterns. The use of the thin Cr layer not only solves the charging effect during the EBL on the insulating substrate, but also facilitates high aspect ratio PhCs by acting as a hard mask while deep etching into the Si x N y . A very high aspect ratio of 10:1 on a 60 nm wide grating structure has been achieved while preserving the quality of the flat top of the narrow lines. The presented nano-fabrication method provides PhC structures necessary for a high quality optical response. Finally, we fabricated a refractive index based PhC sensor which shows a sensitivity of 185 nm per RIU.

Low-cost silver capped polystyrene nanotube arrays as super-hydrophobic substrates for SERS applications.

In this paper, we describe the fabrication, simulation and characterization of dense arrays of freestanding silver capped polystyrene nanotubes, and demonstrate their suitability for surface enhanced Raman scattering (SERS) applications. Substrates are fabricated in a rapid, low-cost and scalable way by melt wetting of polystyrene (PS) in an anodized alumina (AAO) template, followed by silver evaporation. Scanning electron microscopy reveals that substrates are composed of a dense array of freestanding polystyrene nanotubes topped by silver nanocaps. SERS characterization of the substrates, employing a monolayer of 4-aminothiophenol (4-ABT) as a model molecule, exhibits an enhancement factor of ∼1.6 × 10(6), in agreement with 3D finite difference time domain simulations. Contact angle measurements of the substrates revealed super-hydrophobic properties, allowing pre-concentration of target analyte into a small volume. These super-hydrophobic properties of the samples are taken advantage of for sensitive detection of the organic pollutant crystal violet, with detection down to ∼400 ppt in a 2 μl aliquot demonstrated.

Synthesis, optical properties and self-assembly of gold nanorods

Noble metal nanostructures of different aspect ratios were synthesised and optically characterised at individual nanorod level. Rayleigh scattering spectroscopy/scanning electron microscopy measurements were performed to uniquely correlate optical signatures with nanorod size and shape. Scattering spectra of nanorods were dominated by the intense longitudinal surface plasmon resonance (SPR) band in the near-infrared part of the spectrum. This band was found to be highly shape and size dependent. Droplet evaporation techniques and application of dielectrophoretic forces have been used to organise nanorod dispersions into ordered arrays. Depending on the technique and nanoparticle size used, nanorods were found to form one, two or three dimensional (1D, 2D and 3D) superstructures. Within these superstructures nanorods organised themselves into end-to-end lines (1D), side-to-side fashion (2D) or hexagonal arrangements (3D).

Nanofabrication of Robust Nanoelectrodes for Electrochemical Applications

Individual gold nanowire devices for use as nanoelectrodes in electrochemical studies were fabricated using a low-cost robust fabrication approach. Nanowires were characterized by a combination of direct electrical current-voltage measurements and cyclic voltammetry using the model analyte ferrocene monocarboxylic acid. We observed low electrical resistances to contacted nanowires and steady-state sigmoidal electrochemical voltammograms that may be described by classical Butler-Volmer kinetics. The potential of nanowires for use in future biosensing applications was explored by demonstrating mediated bioelectrocatalytic oxidation of glucose.

Novel Single Gold Nanowire-based Electrochemical Immunosensor for Rapid Detection of Bovine Viral Diarrhoea Antibodies in Serum

Bovine Viral Diarrheoa (BVD) is a worldwide disease with severe financial implications for the Bovine beef and dairy industries. A key challenge to BVD eradication is that the requirement to send samples to, and receive results from, specialized laboratories slows down the diagnostic process and leads to uncontrolled spread of the virus within a herd until diagnostic confirmation is received. Consequently, rapid identification of BVD is now critical for herd protection and prevention of costly herd outbreaks and new diagnostic devices, suitable for on-farm analysis, that deliver rapid and early identification of animal disease states, are required. We report, here, an electrochemical onchip fully integrated nanowire based immunosensor device for detection of BVD in serum. The capture biomolecule, BVD virus, is covalently immobilized via a carboxylic terminated polymer firstly electrodeposited onto a single nanowire. Electrochemical characterization including faradaic electrochemical impedance spectroscopy and cyclic voltammetry is performed. Label free immunologic detection of antibodies (10 μg/mL, 20 min) is first demonstrated using a bovine serum albumin as a model antigen-antibody system. Then, the immunosensor is applied to detection of bovine viral diarrhoea antibodies (10 μg/mL, 20 min) in both buffer and serum. The sensor clearly discriminates between positive and negative infected bovine sera. This study clearly shows the potential of this chip nanowirebased electrochemical sensor for immunoassays application in serum with a view to developing portable devices for on-farm diagnosis or therapeutic monitoring in animal health applications.

Highly polarized luminescence from β-phase-rich poly(9,9-dioctylfluorene) nanofibers.

Novel poly(9,9-dioctlylfluorene) (PFO) nanofibers were fabricated by solution template wetting of anodic aluminum oxide (AAO) templates with a pore diameter of 25 nm. Individual nanofibers displayed a pronounced axially polarized luminescence with a typical emission dichroic ratio of 15 and low spread of the emissive species angular distribution. The strong optical characteristics were ascribed to intrachain reorientation of amorphous PFO to a more planar and elongated β-phase conformation induced by mechanical strain during polymer template pore infiltration. Absorption optical spectroscopy on nanofiber mats confirmed formation of 24% β-phase emissive segments, which dominated the nanofiber luminescence characteristics. X-ray diffraction measurements were used to confirm and quantify the extent of nanofiber internal molecular alignment.

Dual resonance approach to decoupling surface and bulk attributes in photonic crystal biosensor.

A sub-wavelength grating-based photonic crystal sensor is designed to excite two spectrally and spatially different guided mode resonances that have distinctive electric field distributions. We present and validate the uni-polarized dual resonance approach to separating bulk index perturbations from surface-binding events in a single measurement by monitoring the resonance wavelength shifts. This self-referencing method will reduce errors in the measurement of biomolecule binding events on sensor surfaces in a perturbed environmental background.