John McPhillips

High-performance biosensing using arrays of plasmonic nanotubes

We show that aligned gold nanotube arrays capable of supporting plasmonic resonances can be used as high performance refractive index sensors in biomolecular binding reactions. A methodology to examine the sensing ability of the inside and outside walls of the nanotube structures is presented. The sensitivity of the plasmonic nanotubes is found to increase as the nanotube walls are exposed, and the sensing characteristic of the inside and outside walls is shown to be different. Finite element simulations showed good qualitative agreement with the observed behavior. Free standing gold nanotubes displayed bulk sensitivities in the region of 250 nm per refractive index unit and a signal-to-noise ratio better than 1000 upon protein binding which is highly competitive with state-of-the-art label-free sensors.

Ultrasensitive Non‐Resonant Detection of Ultrasound with Plasmonic Metamaterials

Ultrasound and photoacoustic imaging have recently been developed for clinical diagnostics and biomedical research. Optical sensors for ultrasound detection provide very high sensitivity and bandwidth, advancing the horizon for the biomedical application of acoustic waves. Here we take advantage of the high sensitivity of plasmonic nanorod metamaterials to variations in the refractive index of their surroundings to demonstrate the ultrasensitive detection of acoustic waves. The measured detection limit is approximately 500 Pa as determined by the signal to noise ratio. The theoretical detection limit of the metamaterial sensor has been shown to exceed that of surface plasmon resonance based sensors in resonant conditions, predicting an ultimate sensitivity of a few tens of Pa. The non-resonant nature, signal linearity, high-bandwidth and sub-nanosecond response time of metamaterial-based sensors make them very promising for state-of-the-art health and biomedical applications. Photoacoustic and ultrasound imaging is widely used in clinical diagnostics and bio-medical research. [ 1 , 2 ] Specialized and emerging ultrasound-based technologies include tissue characterization and image segmentation, microscanning and intravascular scanning, elasticity imaging, refl ex transmission imaging, computed tomography, Doppler tomography and thermo-acoustics, to name a few. [ 3 , 4 ] Most of the recent achievements in ultrasound imaging have been enabled by advances in ultrasound detection technology, [ 2 ] and some signifi cant recent progress has been made employing “acoustic” metamaterials as specially fabricated acoustic lenses that allow much higher, sub-wavelength spatial resolution imaging to be achieved. [ 5 , 6 ] It is widely recognized that both the detection sensitivity and bandwidth are important in order to attain highquality, high-resolution imaging [ 7 , 8 ] and the ultrasound detector

Fabrication and optical properties of large-scale arrays of gold nanocavities based on rod-in-a-tube coaxials

Centimeter sized arrays of gold coaxial rod-in-a tube cavities have been fabricated using anodized aluminum oxide as a template. The etching process used to create the cavities enables the production of extremely small gaps between tube and rod, on the order of 5 nm, smaller than those created by standard fabrication techniques. Normal incidence spectroscopy reveals two extinction peaks in the visible and near infrared wavelength range associated with resonant plasmonic modes excited in the structure. Numerical simulations show that the modes are associated with in-phase and out-of-phase hybridization of transverse dipolar excitations in the nanorod and in the tube.

The controlled fabrication and geometry tunable optics of gold nanotube arrays

Arrays of vertically aligned gold nanotubes are fabricated over several square centimetres which display a geometry tunable plasmonic extinction peak at visible wavelengths and at normal incidence. The fabrication method gives control over nanotube dimensions with inner core diameters of 15-30 nm, wall thicknesses of 5-15 nm and nanotube lengths of up to 300 nm. It is possible to tune the position of the extinction peak through the wavelength range 600-900 nm by varying the inner core diameter and wall thickness. The experimental data are in agreement with numerical modelling of the optical properties which further reveal highly localized and enhanced electric fields around the nanotubes. The tunable nature of the optical response exhibited by such structures could be important for various label-free sensing applications based on both refractive index sensing and surface-enhanced Raman scattering.

Fabrication and optical properties of gold nanowire arrays

In this work the optical properties of arrays of gold nanowires in transmission and attenuated total reflectance (ATR) geometry has been examined further, with a view to maximising the sensitivity by finding optimum nanowire and array dimensions. Gold nanowire arrays were grown by electrochemical deposition into a nano-porous alumina template. The effect of changing the nanowire aspect ratio on the optical properties was investigated.

Resonant electromechanical device fabrication with new thin film materials

Thin film piezoelectric materials have many applications, including in high frequency bulk acoustic wave resonators. However, conventional thin film materials such as AlN have quite poor piezoelectric properties. Research involving bulk piezoelectric material has now produced widely available single crystals such as (1-x)Pb(Mg1/3,Nb2/3)O3-x(PbTiO3) (PMN-PT) which offer much higher performance, and thin films of such materials are also viable. In this paper, work on sputtered AlN and pulsed laser deposited PMN-PT thin films is described, including measurements of Ag/AlN/Al trilayers on several different substrates and Au/PMN- PT/La0.5Sr0.5CoO3 trilayers on single crystal MgO. Analysis via the spacing of parallel resonant frequencies in electrical impedance and S- parameter measurements has also been explored. This technique takes into account that the films are not self-supported and that their properties are dependent on the substrate. Using various software tools most often applied to bulk materials, with appropriate material parameters, further exploration of the suitability of PMN-PT for thin film device fabrication and direct comparison with AlN have been considered.