Designing polymeric nanorod arrays for optical waveguide‐based biosensors

Optical waveguide sensor based on silica nanotube arrays for label-free biosensing.

ZnO/(La, Sr) CoO3 (ZnO/LSCO) core-shell composite nanorod arrays have been successfully synthesized by a sequential combination process of a hydrothermal synthesis followed by a pulsed laser deposition (PLD) process (or a colloidal deposition process). Compared to the colloidal deposition process, PLD produces a more uniform and efficient deposition of continuous and mesoporous LSCO thin films onto ZnO nanorod arrays. During the PLD process, the deposited film uniformity was found to be dependent on the nanorod diameter, array density, and thus specific surface area of the nanorod arrays, in addition to the PLD deposition parameters. Field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used to investigate the surface morphologies and orientations of the composite nanorod arrays. With densely packed ZnO nanorod arrays as a unique support structure, the mesoporous LSCO thin film coated on top exhibited better photocatalytic properties than ZnO nanorod arrays and LSCO thin films deposited on flat Si substrates. With optimization of the structure, dimensionality, packing density, as well as the composition and interface structure, these unique composite nanoarchitectures could be a promising class of photocatalyst candidates for organic molecule degradation.

Layer-by-layer (LbL) deposition of polyelectrolytes and proteins within the cylindrical nanopores of anodic aluminum oxide (AAO) membranes was studied by optical waveguide spectroscopy (OWS). AAO has aligned cylindrical, nonintersecting pores with a defined pore diameter d0 and functions as a planar optical waveguide so as to monitor, in situ, the LbL process by OWS. The LbL deposition of globular proteins, i.e., avidin and biotinylated bovine serum albumin was compared with that of linear polyelectrolytes (linear-PEs), both species being of similar molecular weight. LbL deposition within the cylindrical AAO geometry for different pore diameters (d0 = 25–80 nm) for the various macromolecular species, showed that the multilayer film growth was inhibited at different maximum numbers of LbL steps (nmax). The value of nmax was greatest for linear-PEs, while proteins had a lower value. The cylindrical pore geometry imposes a physical limit to LbL growth such that nmax is strongly dependent on the overall internal structure of the LbL film. For all macromolecular species, deposition was inhibited in native AAO, having pores of d0 = 25–30 nm. Both, OWS and scanning electron microscopy showed that LbL growth in larger AAO pores (d0 > 25–30 nm) became inhibited when approaching a pore diameter of deff,n_max = 25–35 nm, a similar size to that of native AAO pores, with d0 = 25–30 nm. For a reasonable estimation of deff,n_max, the actual volume occupied by a macromolecular assembly must be taken into consideration. The results clearly show that electrostatic LbL allowed for compact macromolecular layers, whereas proteins formed loosely packed multilayers.

Exchange-coupled nanocomposites are considered as the most promising materials for production of high-energy performance permanent magnets, which can exceed neodymium ones in terms of energy product. In this work, micromagnetic simulations of L10-FeNi/SmCo5 composites based on the initially anisotropic structure of nanorods array were performed. Texturing effect on magnetic properties was investigated. It was revealed that even 30% of anisotropy axes misalignment of grains in L10-FeNi phase would lead to only ≈10% drop of coercivity. To maximize magnetic properties of the composites, parameters of microstructure were optimized for 120 × 120 array of interacting nanorods and were found to be 40 nm nanorod diameter and 12–20 nm interrod distance. The estimated diameter of nanorods and the packing density of the array provide energy product values of 149 kJ m−3. Influence of interrod distance on energy product values was explored. Approaches for production of exchange-coupled composites based on anisotropic nanostructures were proposed.

Rutile TiO2 nanorod arrays vertically aligned on conductive fluorine-doped tin oxide glass are optimized for photocatalysis in terms of the density and diameter of nanorods by tuning the concentration of Ti precursor and reaction time during the hydrothermal process. Quantum dots (QDs) of V2VI3( V = Bi; VI = Se, Te) topographic insulators are then employed as sensitizers to enhance the visible-light response of TiO2. Electrochemical measurements show that the decoration of Bi2Se3 or Bi2Te3 significantly increases the photocurrent due to the enhanced light harvesting properties and the charge carrier separation capability mediated by the topographic isolation effect of V2VI3 QDs. Photoelectrocatalytic (PEC) activity evaluation is conducted towards the decoloration of azo dyes, including methyl orange and methylene blue under the sunlight irradiation. The elaborated Bi2Se3/TiO2 nanorod array exhibits the superior PEC performances, presenting shows great potentials in environmental remediation.

Density-controlled zinc oxide (ZnO) nanorod arrays were synthesized on indium tin oxide (ITO) substrates via a potentiostatic electrodeposition method under different conditions. The effects of preparing conditions on the density of nanorods were systematically investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). It is demonstrated that the density and diameter of ZnO nanorods can be effectively controlled by varying the electrodeposition parameters, such as electrodeposition potential, concentration of Zn2+ precursor and growth temperature. Pre-treatment of the substrate also has great influence on the morphology of the electrodeposited ZnO nanorods. By coating the ITO substrates with an gold (Au) or ZnO nanoparticle seed layer, ZnO nanorod arrays with an average diameter of about 50 nm were synthesized, and the nanorod density can be controlled to above 50 x 10(8) rod cm(-2), which is several dozen times greater than that of the ZnO nanorod arrays synthesized on original ITO substrates.

A new type of hydrogel thin film that can be post-modified with biofunctional molecules is investigated for possible applications in evanescent wave optical affinity biosensors. This material is based on a polyisocyanide (PIC) copolymer that self-assembles into a fibrous polymer network. It is covalently tethered to a solid transducer surface, and its swelling gives rise to a hydrogel layer with a thickness up to several micrometers, as characterized by a combination of surface plasmon resonance and optical waveguide spectroscopy. Thin PIC-based hydrogel films with pendant functional biomolecules can be used in label-free affinity sensing strategies. In this work, we demonstrate the PIC-based affinity binding matrix through an avidin coupling of functional biomolecules and immunoassay-based detection, which can be readily expanded to other types of assays and target analytes. In addition, the stable PIC matrix proves to be resistant to fouling, even in the presence of complex biological fluids such as blood plasma or serum.

Phased antenna arrays patterned on optical waveguides can strongly affect mode coupling and propagation in the waveguides. The unidirectional effective wavevector provided by the phased antenna array can facilitate phase matching between different waveguide modes; the effective wavevector can also breaks the symmetry of optical power flow in waveguides. These effects can be used to realize novel integrated photonic circuits. We used full-wave simulations to demonstrate broadband, small-footprint optical waveguide mode converters, polarization converters, and optical power diodes in the telecom and mid-infrared spectral ranges. The designed optical devices have small footprints (i.e., length of antenna arrays ranges from a few to ten times of the free-space wavelength). The efficiency of mode conversion (defined as the percentage of optical power coupled from one waveguide mode to the other) ranges from 10% to 60%, and the purity of converted modes approaches unity. The on-off ratio of optical power diodes (defined as the ratio between optical power transmission in two opposite directions) can be larger than 20dB over a broad wavelength range. We developed a coupled mode theory to model the asymmetric coupling between different waveguide modes and between waveguide modes and surface waves propagating along antenna arrays. The results calculated using the coupled mode theory can successfully explain mode evolution in our waveguide structures observed in simulations. We believe the platform of optical waveguides integrated with phased antenna arrays offers great opportunity to study novel physics originating from interaction between guided waves and designer structures. The platform also has great implication for integrated photonics device applications.

For the first time detailed interactions between optical and acoustic modes in a silicon slot waveguide are presented. A new computer code has been developed by using a full-vectorial formulation to study the acoustic modes in optical waveguides. The results have shown that the acoustic modes in an optical slot waveguide are not purely longitudinal or transverse but fully hybrid in nature. The model allows the effects of Stimulated Brillouin Scattering and the associated frequency shift due to the interaction of these hybrid acoustic modes with the fully hybrid optical mode also to be presented.

A theoretical analysis is given for the scattering and mode conversion of the guided modes in an optical waveguide when cylindrical obstacles of arbitrary cross section are present in the slab waveguide. This structure is a complicated inhomogeneous media caused by melt‐connection of the optical fibers during fabrication. When a guided mode enters such a non‐uniform path, the equivalent surface electromagnetic current at the dielectric surface can be found by the successive approximation method which is applicable to the integral equation in which current is an unknown variable. Thus the mode‐conversion power factor and the scattering power pattern of the scattered wave can be determined. As a numerical example, the relationships between the shape and size of the non‐uniform portion is given for the TE mode.

Vertically aligned copper indium diselenide nanorod arrays were synthesized by electrodeposition using porous anodic alumina template supported on a tungsten/silicon substrate. Porous anodic alumina template was fabricated by anodizing aluminium film which was sputtered onto tungsten/silicon substrate. The approach employed a selective chemical etching to penetrate the barrier layer at the bottom of alumina pore before electrodeposition. It enables direct electrical and chemical contact with the underside substrate electrode. The as-deposited sample was annealed at 450 °C in vacuum. High resolution transmission electron microscopy and x-ray diffraction showed high purity CuInSe2 nanorods with a preferred [112] orientation. Scanning electron microscopy revealed that the nanorods were dense and compact with diameter about 100 nm and with length approximate 1000 nm. Energy-dispersived X-ray spectroscopy showed fabrication of nearly close to stoichiometric CuInSe2. The energy band gap of this nonorod arrays was analysed by fundamental absorption spectrum and evaluated to be 1.06 eV.

We present a new approach to produce ZnO nanorods in a reproducible manner at a temperature lower than other physical vapour deposition techniques, such as the vapour–liquid–solid mechanism. Arrays of well-aligned ZnO nanorods of uniform diameter have been synthesized on the Si substrate precoated with Au, using a simple electron beam evaporation method without the flow of any carrier gas. Scanning electron microscopy and atomic force microscopy characterizations show that as-grown nanorods are well aligned and uniform in diameter. X-ray diffraction measurements and clear lattice fringes in high-resolution transmission electron microscopy image show the growth of good quality polycrystalline hexagonal ZnO nanorods and a ⟨0 0 2⟩ growth direction. The polarization-dependent studies of near edge x-ray absorption fine structure (NEXAFS) are performed to investigate the electronic structure of the zinc and oxygen ions. The analysis of NEXAFS spectra at different angles of incidence of photon flux indicates the formation of ZnO nanorods having anisotropic behaviour of O and Zn states. The photoluminescence spectrum exhibits strong ultraviolet emission at 385 nm and the UV–visible spectrum also shows a band-gap transition around 390 nm indicating the good quality of nanorods. The catalytic growth mechanism of the ZnO nanorods is discussed on the basis of experimental results in this work.

A new technique using optical waveguides has been developed for studying the effects of moderate to high power laser radiation on cells [1]. A monolayer of attached cells covering a wide area on the waveguide surface is exposed to the evanescent tail produced by the propagating waveguide mode. The optical radiation-cell interaction is limited to involve only the high power density and intense electric field strength associated with the evanescent tail. It is well established that extremely high power densities (105 to 108 watts/ cm2)are obtained in the waveguide when a waveguide mode is excited by a moderate power (1–2 watts), continuous wave (CW) laser [2–3]. These optical power densities create intense electric fields at optical frequencies ranging from 103 to 105 volts/cm within the waveguide. The evanescent tail of such a waveguide mode extends into the medium above the waveguide and creates an intense electric field at the interface between the waveguide and the medium. By adjusting the input laser power and waveguide parameters, the electromagnetic field intensity and distance of penetration into the cell of the evanescent tail can be varied over a wide range. The importance of intense electromagnetic field induced kinetic effects and photochemical reactions in producing lethal effects in cells has been stressed by previous investigators [4–6]. This report describes the reduced survival measured for monolayers of EMT-6 mammary sarcoma cells exposed to the evanescent tail created in a waveguide.

We demonstrate control of ZnO nanorod density for self-organized growthon ZnO buffer layers on Si by varying Zn supersaturation during theinitial growth phase, thereby altering the competition between 2D and 1Dgrowth modes. Higher initial supersaturation favours nanorods of diameter<200 nm with micron-sized bases, resulting in low density nanorod arrays;nanorods grown with lower initial supersaturation have diameters<200 nm along their entire length, yielding higher density arrays. Fieldemission and imaging studies reveal field enhancement factors of>1000, attributed to sharp facet edges, and indicate that lower density arrays have more uniformemission due to a reduction in screening effects.

Nanoporous media scatter a small fraction of the light propagating through them, even if pore sizes are significantly smaller than the characteristic visible wavelengths. The disordered spatial modulation of the refractive index at the few or few tens of nanometers length scale, resulting from the presence of randomly distributed air bubbles or solid aggregates within a continuous solid background, gives rise to these weak scattering effects. However, standard theoretical approaches to describe this kind of media use effective medium approximations that do not account for diffuse, ballistic, and specular components. Herein, all spectral components and the angular distribution of the scattered light are captured through optical modeling. A Monte Carlo approach, combining scattering Mie theory and Fresnel equations, implemented within a genetic algorithm, allows us to decode the void and aggregate size distribution and hence the internal structure of a nanocrystalline titania (TiO2) film chosen as a paradigmatic example. The approach allows to generically describe the scattering properties of nanoporous materials which, as shown herein, may be used to decipher their internal structure from the fitting of their far‐optical field properties.