Aligned Gold Nanorods Research Articles

Self‐Assembly of Vertically Aligned Gold Nanorod Arrays on Patterned Substrates

Dicht gepackte Nanostäbe: Ein Selbstorganisationsprozess auf der Basis von Konvektions- und Kapillarkräften wurde genutzt, um lithographisch vorgegebene Bereiche mit Anordnungen aufrecht stehender Goldnanostäbe zu versehen (siehe Bild). Die hexagonal dichteste Packung der Goldnanostäbe erzeugt ein ideales Substrat für die oberflächenverstärkte Raman-Spektroskopie.

Self‐Assembly of Vertically Aligned Gold Nanorod Arrays on Patterned Substrates

Self‐Assembly of Vertically Aligned Gold Nanorod Arrays on Patterned Substrates

Polymer Nanofibers Embedded with Aligned Gold Nanorods: A New Platform for Plasmonic Studies and Optical Sensing

This paper reports the fabrication and characterization of polymer nanofibers embedded with gold nanorods in uniaxial alignment for applications in optical waveguiding and sensing. Using a waveguiding approach, we demonstrated highly efficient excitation of localized surface plasmon resonance in the embedded gold nanorods with a photon-to-plasmon-conversion efficiency as high as 70% for a single nanorod at its longitudinal resonance wavelength. On the basis of waveguiding polymer nanofibers embedded with gold nanorods, we further demonstrated compact optical humidity sensors with a response time of 110 ms and an operation optical power as low as 500 pW.

Templating of Self-Alignment Patterns of Anisotropic Gold Nanoparticles on Ordered SWNT Macrostructures

We report a simple and versatile technique for oriented assembly of gold nanorods on aligned single-walled carbon nanotube (SWNT) macrostructures, such as thin nanotube films and nanotube fibers. The deposition and assembly is accomplished via drop drying of dilute gold nanorod suspensions on SWNT macrostructures under ambient conditions. Guided by anisotropic interactions, gold nanorods, and polygonal platelets spontaneously align with SWNTs, resulting in macroscopic arrays of locally ordered nanorods supported on aligned SWNT substrates. SEM reveals that the scalar order parameter of rods relative to the local average SWNT alignment is 0.7 for rods on SWNT films and 0.9 for rods on SWNT fibers. This self-alignment is enabled by anisotropic gold nanoparticle-SWNT interactions and is observed for a wide range of nanoparticles, including nanorods with aspect ratios ranging from 2-35, thin gold triangular and other polygonal platelets. The plasmonic properties of aligned gold nanorods together with superior electronic, chemical and mechanical properties of SWNTs make these hybrid nanocomposites valuable for the design of self-assembled multifunctional optoelectronic materials and optical metamaterials.

Polarization-Dependent Surface-Enhanced Raman Scattering via Aligned Gold Nanorods in Poly(Vinyl Alcohol) Film

A low-cost and effective surface-enhanced Raman scattering (SERS) substrate consisting of aligned gold nanorods is obtained by stretching the poly(vinyl alcohol) nanorods composite film doped with the probe molecule. The SERS intensity of characteristic vibrational band of the probe is observed obviously dependent on the angle (θ) between incident polarization and major axis of nanorod. The relationship between them manifests a cos2θ dependence. The result is illustrated from both the localized field enhancement and optical antenna effect of gold nanorod. The finite element method calculation is also performed to further confirm the conclusion.

Brownian Fluctuations and Heating of an Optically Aligned Gold Nanorod

We present the first quantitative measurements of the torque exerted on a single gold nanorod in a polarized three-dimensional optical trap. We determined the torque both by observing the time-averaged orientation distribution and by measuring the dynamics of the rotational brownian fluctuations. The measurements are in good agreement with calculations, where the temperature profile around the hot nanorod gives rise to a reduced, effective viscosity. The maximum torque on a 60 nm×25 nm nanorod was 100 pN·nm, large enough to address single-molecule processes in soft and biological matter.

Directed assembly of gold nanorods using aligned electrospun polymer nanofibers for highlyefficient SERS substrates

Nonspherical metal nanoparticles are very attractive plasmonic nanostructures owing to thefacile tunability of the plasmonic properties and the presence of sharp corners and edges,which act as electromagnetic hot spots for surface enhanced Raman scattering(SERS). However, such anisotropic nanostructures exhibit strong polarizationdependence in their plasmonic properties, exhibiting significantly higher SERSintensity in certain orientations. In this paper, we demonstrate a facile strategy toachieve directed assembly of aligned gold nanorods using highly aligned electrospunnanofibers. We believe that the interstices between the nanofibers act as micro-andnanochannels, resulting in hydrodynamic drag forces on the gold nanorods, thusinducing massive alignment of the same on the nanofibers. Apart from exhibitingnearly 50 times higher SERS intensity compared to a planar SERS substrate withrandomly oriented nanorods, our results highlight the importance of the orientation ofanisotropic nanostructures. Finite difference time domain (FDTD) simulationsemployed to understand the electromagnetic field distribution around an alignednanorod array showed excellent agreement with the experimental observations.

Wavelength Dependence of Raman Enhancement from Gold Nanorod Arrays: Quantitative Experiment and Modeling of a Hot Spot Dominated System

Surface-enhanced Raman scattering (SERS) spectra from molecules adsorbed on the surface of vertically aligned gold nanorod arrays exhibit a variation in enhancement factor (EF) as a function of excitation wavelength that displays little correlation with the elastic optical properties of the surface. The key to understanding this lack of correlation and to obtaining agreement between experimental and calculated EF spectra lies with consideration of randomly distributed, sub-10 nm gaps between nanorods forming the substrate. Intense fields in these enhancement “hot spots” make a dominant contribution to the Raman scattering and have a very different spectral profile to that of the elastic optical response. Detailed modeling of the electric field enhancement at both excitation and scattering wavelengths was used to quantitatively predict both the spectral profile and the magnitude of the observed EF.

Anisotropic and enhanced absorptive nonlinearities in a macroscopic film induced by aligned gold nanorods

Anisotropic and enhanced nonlinear absorption (NLA) has been observed from aligned gold nanorods (GNRs) embedded in a poly(vinyl alcohol) film, which was realized by a stretched-film method. Open-aperture Z-scan experiments revealed that the stretch process enhanced the NLA coefficient by approximately nine times and increased the anisotropic factor of NLA to ∼20. The enhancement in the NLA coefficient reached as high as ∼91 times after increasing the concentration of GNRs by four times and this is attributed to the plasmonic interaction between densely packed GNRs.

Strong Surface Plasmon Resonance of Ordered Gold Nanorod Array Fabricated in Porous Anodic Alumina Template

High-density, uniform-sized and vertically aligned gold nanorods were grown on aluminum substrate by DC electrodeposition into modified porous anodic alumina (PAA) template. Optical reflection measurements using s- and p-polarized light showed strong surface plasmon resonances (SPRs), for both Au/PAA composites and freestanding Au nanorod arrays. By changing the aspect ratio of the Au nanorods, the angle of incidence of the polarized light, and the dielectric environment, it was possible to vary the position and the intensity of the SPR reflection minima in a reproducible and predictable manner. We successfully measured higher order transverse SPR, which proves the formation of highly uniform Au nanorods.

Density-Dependent Electrochemical Properties of Vertically Aligned Gold Nanorods

Gold nanorods (AuNRs) with an aspect ratio of 2.33 or 3.16 were self-assembled onto 1,6-hexanedithiol-modified gold electrodes based on covalent interaction at a solution temperature of 35 °C. The formation of the 1,6HDT/AuNR bilayers as a function of the nanorods’ adsorption time was studied by atomic force microscopy and quartz crystal microbalance, whereas their physical properties and chemical bonding were studied by contact angle and FT-IRRAS spectroscopy measurements. It was found that both types of nanorods were covalently bonded to the Au-1,6HDT-SAM modified electrodes in an end topography and with a high surface density. The electrochemical properties of the Au-1,6HDT-AuNR modified electrodes, as a function of the nanorods’ adsorption time, were studied by cyclic voltammetry, square wave voltammetry, and electrochemical impedance spectroscopy using [Fe(CN)6]3−/4−as the redox probes. The highest enhancement of the electrical current in the cyclic voltammograms was recorded at the Au-1,6HDT-AuNR modified electrodes for 7 h of chemisorption of 2.33 aspect ratio rods or 15 h of chemisorption of 3.16 aspect ratio rods. The high decrease of the apparent charge-transfer resistance upon nanorod self-assembly suggests a charging of the rods by the [Fe(CN)6]3−/4− in solution and electron transfer across them. Moreover, the variation of the tunneling parameter β suggests that the electron tunneling process through the 1,6HDT molecules is more efficient at the electrodes modified with bilayers containing short rods (β = 0.78 ± 0.08 Å−1/per methylene unit) than at the electrodes modified with bilayers containing long rods (β = 0.84 ± 0.10 Å−1/per methylene unit). The self-assembly of the AuNRs in an end-bonding topography with a high surface coverage restored almost completely the electronic communication that was entirely blocked by the preceding 1,6HDT layer.

Local field calculations of the anisotropic nonlinear absorption coefficient of aligned gold nanorods embedded in silica

The anisotropic nonlinear third-order susceptibility of nanocomposites consisting of aligned ellipsoidal metallic nanoparticles embedded in a dielectric matrix is modeled from the generalized Maxwell-Garnett equation involving depolarization factors. Depolarization factors take into account different anisotropic particle geometries such as flat disks, rods, or ellipsoids. The equations traditionally used to model third-order susceptibility of nanocomposites are valid only for very low metal volume fractions. Modified equations that allow metal volume fractions up to the limit of validity of the Maxwell-Garnett equation are used. The effect of the different model parameters, namely, the metal volume fraction, the real and imaginary parts of the metal dielectric constant, the matrix dielectric constant, and, finally, the ratio of the real and imaginary parts of the metal third-order susceptibility were investigated using the model gold/silica nanocomposite system. As previously reported in the literature for the isotropic particle case, counterintuitive effects such as sign reversal between the bulk metal and composite nonlinear susceptibilities have been observed. The calculations were applied to the case of gold nanorods embedded in silica that were experimentally found to exhibit anisotropic saturable absorption.

Visible Laser Desorption/Ionization Mass Spectrometry Using Gold Nanorods

In this report, we demonstrate the novel application of gold nanorods in the laser desorption/ionization mass spectrometry. The gold nanorod substrate was fabricated by electrodepositing the gold into the pores of the porous alumina template. The embedded nanorods with diameter of ∼15 nm were exposed to the surface by partial removal of the alumina template. The vertically aligned gold nanorods were irradiated by a frequency-doubled/-tripled Nd:YAG laser and the desorption/ionization was found to be favored by the use of 532 nm visible laser, which is in the range of the localized surface plasmon resonance. The present technique offers a potential analytical method for the low-molecular-weight analytes which are rather difficult to handle in organic matrix-assisted laser desorption/ionization mass spectrometry.

Directing the Growth of Highly Aligned Gold Nanorods through a Surface Chemical Amidation Reaction

This paper describes the seed-mediated growth of highly aligned gold nanorods (Au NRs) over large areas directly on a Si(100) surface. The Si(100) surface is NH2-functionalized with (aminopropyl)triethoxysilane (APTES) followed by a DCC-catalyzed surface amidation reaction with acetic acid. After exposure to a gold nanoparticle (Au NP) "seed" solution, chemical seed-mediated growth of the surface-bound seeds via reduction of AuCl4- by ascorbic acid in the presence of cetyltrimethylammonium bromide leads to the growth of highly aligned Au NRs on the surface. About 80% of the NRs are aligned in the same direction within a +/-30 degrees range. Au NRs account for 19% of the nanostructures with average aspect ratio (AR) of approximately 20. The alignment direction did not correlate with the atomic arrangement of the Si(100) crystal since it varied over different regions of the sample, rotating by 90 degrees from top to bottom of an approximately 5 mm sample. Si crystallinity may still be important since alignment is not observed on amorphous glass. Surface functionalization is the key since alignment is only observed following the amidation reaction and not on NH2-functionalized, SH-functionalized, or bare Si(100) surfaces. Alignment also occurred for Au NRs grown on Si(100)/APTES reacted with succinic acid and on Ag NRs grown on Si(100)/APTES/acetic acid surfaces. This unique alignment of metal NRs promoted by a surface amidation chemical reaction may find use in nanoelectronics, chemical sensing, and plasmonics applications.

Alignment of Gold Nanorods in Polymer Composites and on Polymer Surfaces

Alignment of Gold Nanorods in Polymer Composites and on Polymer Surfaces

Aligned Gold Nanorods in Silica Made by Ion Irradiation of Core–Shell Colloidal Particles

Colloidal particles with a 14 nm diameter Au core surrounded by a 72 nm thick silica shell have been irradiated with 30 MeV heavy ions. The shell deforms into an oblate ellipsoid, while the core becomes rod‐shaped (aspect ratio up to 9) with the major axis along the beam. Optical extinction measurements show evidence for split plasmon bands, characteristic for anisotropic metal nanoparticles.