Gold Nanorod Arrays Research Articles

Hierarchical assembly of gold nanorod stripe patterns for sensing and cells alignment

Hierarchical assemblies of nanomaterial superstructures with controlled orientation affords a multitude of novel properties of plasmonics and broad applications. Yet constructing multi-functional superstructures with nanoparticles positioned in desired locations remains challenging. Herein, gold nanorods (GNRs) assembled in stripe patterns with controlled orientation and structures in millimeter scale for versatile application have been achieved. Applications of patterned GNRs in sensing enhancement and engineering mammalian cells alignment are investigated experimentally. The performance of patterned GNRs in surface enhanced Raman scattering (SERS) and electrical sensing are found in orientational dependence. The SERS signals of vertically arranged GNR arrays exhibit double the folder intensity than those horizontally arranged. In contrast, the horizontally arranged GNRs exhibit twice as much electrical conductivity. The system is further explored to pattern mammalian cells. For the first time, we reveal the nanostructured topography of GNR confined cells to a specific region, and direct the adhesion and extension of living cells, which opens up broad applications in tissue engineering and biosensing.

Plasmon Coupling Within the Multifold Nanorod Metasurface for Sensing Applications

A novel design for improving the sensing performance of localized surface plasmon resonance based nanosensors is proposed. We analyze the Fano-like coupling interaction within the multifold gold nanorod metasurface. By means of such Fano-like coupling effects, we can enhance the figure of merit (FoM) of nanosensors based on gold nanorod array by nearly one order. The main idea of our design can be easily adjusted for other nanophotonic platforms, and provides new possibilities for improving the performance of a myriad of applications, such as slow light devices, and optical switches.

Fabrication of gold nanorods for SERS detection of thiabendazole in apple

Thiabendazole (TBZ) is a kind of pesticide that is widely used in agriculture, and its residue may pose a threat to human health. In order to measure TBZ residues in food samples, a surface-enhanced Raman spectroscopy (SERS) method combined with a homogeneous and reusable gold nanorods (GNR) array substrate was proposed. GNR with a high uniformity was synthesized and then applied to the self-assembly of a GNR vertically aligned array. The relative standard deviation (RSD) of the array for SERS could reach 15.4%, and the array could be reused for more than seven times through the treatment of plasma etching. A logarithmic correlation between TBZ concentration and Raman intensity was obtained, with the best determination coefficient (R2) and the corresponding limit of detection (LOD) of 0.991 and 0.037 mg/L in methanol solution, and 0.980 and 0.06 ppm in apple samples, respectively. The recoveries of TBZ in apple samples ranged from 76% to 107%. This study provided a rapid and sensitive approach for detecting TBZ in apples based on SERS coupled with GNR array substrate, showing great potential for analyzing other trace contaminants in food matrices.

On-substrate porous-anodic-alumina-assisted gold nanostructure arrays: Meeting the challenges of various sizes and interfaces

Arrays of 0- and 1-dimensional noble-metal nanostructures aligned on solid substrates are in demand for nanocatalysis, bio- and optical sensing, or biomolecular analysis. Here we introduce a range of advances based on a systematic research towards the porous-anodic-alumina (PAA)-assisted on-substrate arrays of gold nanostructures, such as rods and spheres, spatially-separated and highly aligned on a metal or semiconductor supporting layer via a blend of the anodizing, re-anodizing, and post-anodizing treatments applied to a thin layer of Al superimposed on selected valve metals (W, Ti, Hf), metal bilayers (W/Ti), or binary metal alloy layers (W-Ti). The achievements are due to (1) the improved self-organization in the PAA thin films during the self-localizing high-current anodization of the upper Al layer at challenging potentials ranging 100–250 V and 20 to 5 V, and (2) the enhanced penetration of the alumina barrier layer by the undergrowing metal oxide due to the increasing polarization (re-anodizing). The protrusions of the undergrown metal oxide can be either selectively dissolved away providing perfect nanoholes in the alumina barrier layer or left as formed in the barrier layer and annealed in vacuum to increase their electron conductance and serve as the supports for subsequent metal electrodeposition. Additionally, the in-situ amplitude-modulated constant-current pulse deposition mode combined with the original surface-wiping technique to remove the overdeposited gold allow for smooth nucleation and uniform finishing of perfect arrays of on-substrate gold nanospheres and nanorods, having diameters from 10 to over 250 nm and length up to 2.5 μm.

Förster Resonance Energy Transfer inside Hyperbolic Metamaterials

The ability to control Forster resonance energy transfer (FRET) between emitters via the design of nanostructured materials with appropriate electromagnetic properties is important in the development of fast and enhanced sources of illumination, high-efficiency photovoltaic devices, and biomedical applications, such as nanorulers. While the engineering of the local density of states allows an efficient control over the spontaneous emission rate, its influence on the FRET process has been an ongoing debate and has led to disparate experimental and theoretical results. Here, we experimentally demonstrate an increase in the FRET rate for donor–acceptor (D–A) pairs separated by fixed distances (3.4, 6.8, and 10.2 nm) located inside a hyperbolic metamaterial comprised of an array of gold nanorods. While the modification of the local density of states surrounding the D–A pairs strongly influences the FRET rate, leading to a 13-fold increase inside the metamaterial, the FRET efficiency is shown to remain mostly ...

Systematic Analysis of the Coupling Effects within Supported Plasmonic Nanorod Antenna Arrays

Vertically aligned gold nanorod arrays (of aspect ratios from 3.0 to 18.0) supported on metal substrates were fabricated by electrochemical deposition within porous anodic aluminum oxide templates. However, the coupling effects that occur within these supported arrays are complex, involving both particle–substrate and particle–particle coupling, and far from fully understood. We have performed a systematic investigation into these effects using finite element modeling and correlated these results to experiment. We demonstrate that within the strong coupling regime, the optical properties of the arrays are predominantly governed by inter-rod spacing. Additionally, by supporting the arrays on metal films, the absorption efficiency is significantly enhanced. We explain these coupling effects in terms of plasmon hybridization theory and image charges. We demonstrate that the longitudinal mode may be tuned throughout the visible region and present resonant wavelength contour plots as a function of inter-rod sp...

Magneto-optical effects in hyperbolic metamaterials.

Highly anisotropic metal-dielectric structures reveal unique dispersion properties providing new optical effects. Here we study experimentally linear optical and magneto-optical response of arrays of plasmonic gold nanorods and similar structures complemented by a thin nickel film. We show that both types of structures reveal distinct optical features expected for hyperbolic media and associated with the epsilon-near-zero (ENZ) and epsilon-near-pole (ENP) points. In the case of Ni-containing nanocomposites, we observe linear magneto-optical effects in transmission through the structure, increasing in the vicinity of these points. This observation reveals an important role of the local field enhancement in a hyperbolic medium associated with ENZ and ENP dispersion points in the appearance of magneto-optical activity of magnetic hyperbolic metamaterials.

Fabrication of Large-Area Arrays of Vertically Aligned Gold Nanorods.

Anisotropic nanoparticles, such as nanorods and nanoprisms, enable packing of complex nanoparticle structures with different symmetry and assembly orientation, which result in unique functions. Despite previous extensive efforts, formation of large areas of oriented or aligned nanoparticle structures still remains a great challenge. Here, we report fabrication of large-area arrays of vertically aligned gold nanorods (GNR) through a controlled evaporation deposition process. We began with a homogeneous suspension of GNR and surfactants prepared in water. During drop casting on silicon substrates, evaporation of water progressively enriched the concentrations of the GNR suspension, which induces the balance between electrostatic interactions and entropically driven depletion attraction in the evaporating solution to produce large-area arrays of self-assembled GNR on the substrates. Electron microscopy characterizations revealed the formation of layers of vertically aligned GNR arrays that consisted of hexagonally close-packed GNR in each layer. Benefiting from the close-packed GNR arrays and their smooth topography, the GNR arrays exhibited a surface-enhanced Raman scattering (SERS) signal for molecular detection at a concentration as low as 10-15 M. Because of the uniformity in large area, the GNR arrays exhibited exceptional detecting reproducibility and operability. This method is scalable and cost-effective and could lead to diverse packing structures and functions by variation of guest nanoparticles in the suspensions.

Fabrication of a Vertically Aligned Au Nanorod Array via Block‐Copolymer‐Templated Electroplating

AbstractHexagonally assembled gold (Au) nanorod arrays oriented perpendicular to the substrate are fabricated by block‐copolymer‐templated electroplating. An amphiphilic diblock copolymer consisting of poly(ethylene oxide) (PEO) and poly(methacrylate) bearing an azobenzene mesogen on the side chain (PEO‐b‐PMA(Az)) thin film is used as a template without any nanopore‐generation processes. The PEO within the microphase‐separated cylindrical domains of the template function as an electrolytic medium to electroplate Au. The Au nanorods selectively grow through the PEO cylindrical domains. The growth process of the Au nanorods is monitored using cyclic voltammetry and field emission scanning electron microscopy. The feature size (diameter, periodicity, and length) of the Au nanorods is controlled by the feature size and thickness of the template and the amount of passed charge. The optical characteristics of the resulting Au nanorod array are carefully investigated, including angle‐dependent ultraviolet–visible–near infrared spectra and surface enhanced Raman scattering activity. In addition, the morphology of Au nanorods in the template change into Au nanoparticles based on Rayleigh instability.

Highly Sensitive and Reproducible SERS Sensor for Biological pH Detection Based on a Uniform Gold Nanorod Array Platform.

Conventional research on surface-enhanced Raman scattering (SERS)-based pH sensors often depends on nanoparticle aggregation, whereas the variability in nanoparticle aggregation gives rise to poor repeatability in the SERS signal. Herein, we fabricated a gold nanorod array platform via an efficient evaporative self-assembly method. The platform exhibits great SERS sensitivity with an enhancement factor of 5.6 × 107 and maintains excellent recyclability and reproducibility with relative standard deviation (RSD) values of less than 8%. On the basis of the platform, we developed a highly sensitive bovine serum albumin (BSA)-coated 4-mercaptopyridine (4-MPy)-linked (BMP) SERS-based pH sensor to report pH ranging from pH 3.0 to pH 8.0. The intensity ratio variation of 1004 and 1096 cm-1 in 4-MPy showed excellent pH sensitivity, which decreased as the surrounding pH increased. Furthermore, this BMP SERS-based pH sensor was employed to measure the pH value in C57BL/6 mouse blood. We have demonstrated that the pH sensor has great advantages such as good stability, reliability, and accuracy, which could be extended for the design of point-of-care devices.

Coupled Plasmon Resonances and Gap Modes in Laterally Assembled Gold Nanorod Arrays

Abstract The assembly of metal nanocrystals offers a flexible method for creating new materials with tunable, size-dependent optical properties. Here we study the lateral assembly of gold nanorods into arrays, which leads to strong colour changes due to surface plasmon coupling. We also demonstrate the first example of gap modes in colloid systems, an optical mode in which light waves propagate in the channels between the gold rods. Such modes resonate at wavelengths which strongly depend on the gap width and length.

Estimation of polarization distribution on gold nanorods system from hierarchical features of optical near-field

To design optoelectronic functionalities in nanometer scale based on interactions of electronic system with optical near-fields, it is essential to evaluate the relationship between optical near-fields and their sources. Several theoretical studies have been performed, so far, to analyze such complex relationship to design the interaction fields of several specific scales. In this study, we have performed detailed and high-precision measurements of optical near-field structures woven by a large number of independent polarizations generated in the gold nanorods array under laser light irradiation at the resonant frequency. We have accumulated the multi-layered data of optical near-field imaging at different heights above the planar surface with the resolution of several nm by a STM-assisted scanning near-field optical microscope. Based on these data, we have performed an inverse calculation to estimate the position, direction, and strength of the local polarization buried under the flat surface of the sample. As a result of the inverse operation, we have confirmed that the complexities in the nanometer scale optical near-fields could be reconstructed by combinations of induced polarization in each gold nanorod. We have demonstrated the hierarchical properties of optical near-fields based on spatial frequency expansion and superposition of dipole fields to provide insightful information for applications such for secure multi-layered information storage.

Self-assembled vertically aligned Au nanorod arrays for surface-enhanced Raman scattering (SERS) detection of Cannabinol

Self-assembled multi-layered vertically aligned gold nanorod (AuNR) arrays have been fabricated by a simple preparation process that requires a balance between the particle concentration and the ionic strength of the solvent. An experimentally determined critical AuNR concentration of 2.0nM and 50mM NaCl produces well-ordered vertically aligned hexagonally close-packed AuNR arrays. We demonstrate surface treatment via UV Ozone cleaning of such samples to allow introduction of analyte molecules (benzenethiol and cannabinol) for effective surface enhanced Raman scattering detection. This is the first demonstration of the SERS analysis of cannabinol. This approach demonstrates a cost-effective, high-yield and simple fabrication route to SERS sensors with application in the screening for the cannabinoids.

Macroscopic Assembly of Gold Nanorods into Superstructures with Controllable Orientations by Anisotropic Affinity Interaction

Two-dimensional or three-dimensional highly ordered arrays of anisotropic nanoparticles provide attracting properties that are highly desired by the industry. Traditional assembly methods such as evaporation usually produces the nanostructure arrays only up to the millimeter scale with poor control of nanoparticle orientation, making them hardly applicable for industrial needs. Here, we report a facile method to assemble centimeter-scale gold nanorod (Au NR) arrays with highly controlled nanoparticle orientation and high reproducibility. We selectively functionalized the transverse or longitudinal facets of Au NRs with polyethylene glycol (PEG) molecules and utilized the interfacial polymeric affinity between the PEG domains on Au NRs and the PEGylated substrate to achieve the anisotropic self-assembly. The side-PEGylated Au NRs formed closely packed horizontal arrays, whereas the end-PEGylated Au NRs formed vertically standing arrays on the substrate, respectively. The obtained Au NR arrays with different orientations showed anisotropic surface-enhanced Raman scattering (SERS) performance. We showed that the vertically ordered Au NR arrays exhibited 3 times higher SERS signals than the horizontally ordered arrays.

Highly Aligned Plasmonic Gold Nanorods in a DNA Matrix

AbstractSince the Lycurgus Cup was made in the 4th century, metal nanoparticles have attracted much interest due to the characteristics of the plasmonic and metamaterials that show beautiful colors. Despite these fascinating properties, the practical use is limited because it is difficult to control the orientation of the plasmonic nanoparticles. Here, highly aligned plasmonic gold nanorods are obtained using self‐assembled DNA material. Simple mechanical shearing results in long‐range DNA–gold nanorod arrays which show parallel, perpendicular, and zigzag configurations due to the competition between the shear force and DNA elasticity. The resulting surface plasmonic resonance properties of the aligned DNA–gold nanorods film show highly polarization‐dependent behavior in a large area, which is critical for optical and photonic applications. This simple way to form anisotropic plasmonic films can be used for plasmonic nanoparticles in potential applications such as displays and sensors.

Spontaneous Emission inside a Hyperbolic Metamaterial Waveguide

The ability to control the rate of spontaneous emission via the design of nanostructured materials with appropriate electromagnetic properties is important in the development of novel fast sources of incoherent illumination, single photon emitters for quantum optical applications, laser physics and de-excitation of electronic states leading to photodegradation in organic materials. Here, for an emitter placed inside a hyperbolic metamaterial slab of finite thickness comprised of an array of gold nanorods, we experimentally demonstrate an enhancement of the fluorescence coupled to the waveguided plasmon-polariton modes of the metamaterial. We show that fluorescence properties in such finite-size metamaterial design behave differently from commonly studied infinite metamaterials or when the emitters are placed near the metamaterial interface. The emitters inside the metamaterial waveguide exhibit an almost 50-fold reduction of their lifetime, whereas a much smaller reduction (a factor of 2-3) is observed fo...

Fabrication and Characterisation of an Adaptable Plasmonic Nanorod Array for Solar Energy Conversion

The surface plasmonic modes of a side-by-side aligned gold nanorod array supported on a gold substrate has been characterised by electron energy loss spectroscopy (EELS). Plasmonic coupling within the array splits the nanorods’ longitudinal mode into a bright mode (symmetrically aligned dipoles) and a dark mode (anti-symmetrically aligned dipoles). We support this observation by means of finite element modelling (FEM).

Hybrid Organic-Plasmonic Nanoantennas with Enhanced Third-Harmonic Generation.

Resonantly excited plasmonic gold nanoantennas are strong sources of third-harmonic (TH) radiation. It has been shown that the response originates from large microscopic nonlinearity of the gold itself, which is enhanced by the near-field of the plasmonic nanoantenna. To further enhance this response, one can incorporate nonlinear media into the near-fields of the nanoantenna, as an additional TH source. To obtain a significant contribution from the added medium, its nonlinear susceptibility should be comparable to that of the antenna material. Many organic materials offer the necessary nonlinear susceptibility and their incorporation is possible with simple spin-coating. Furthermore, organic materials are often susceptible to photodegradation. This degradation can be used to investigate the influence of organic materials on the hybrid system. Our investigated hybrid organic plasmonic nanoantenna system consists of a gold nanorod array and poly(methyl methacrylate) as the nonlinear dielectric medium. The experiments clearly reveal two contributions to the generated TH radiation, one from the nanoantenna itself and one from the polymer. The nonlinear response of the hybrid material exceeds the response of both individual constituents and opens the path to more efficient nanoscale nonlinear light generation.

DNA Brush-Directed Vertical Alignment of Extensive Gold Nanorod Arrays with Controlled Density.

Control over the orientation of metal nanorods is important for both fundamental and applied research. We show that gold nanorods (GNRs) can be aligned in a single direction by adsorbing positively charged GNRs onto a double-strand DNA-grafted substrate through electrostatic interaction. The ordered structure can be optimized by controlling the density of the positive charges on the surface of the GNRs. We found, in agreement with the results of theoretical simulation, that the resultant structure exhibits plasmonic properties that are dependent on the GNR orientation relative to the direction of an oscillating electric field. Our approach provides new insights into the polymer-assisted self-assembly of rod-shaped nanoparticles utilizing electrostatic interactions.

Mechanical tuning of plasmon resonances in elastic, two-dimensional gold-nanorod arrays

Two-dimensional gold-nanorod arrays (2D-GNA) exhibit distinct resonance peaks in the visible wavelength range that are clearly associated with long and short axis plasmon oscillations. In this paper, we demonstrate a flexible and reproducible way for controlling the plasmon resonance of such 2D-GNAs in-situ, even post-fabrication process, simply by embedding free-standing nanorod arrays into an elastomer thin film. Stretching the polymer film shows the plasmon long-axis resonance to red-shift proportionally to the applied force by as much as 20~nm by increasing the center-to-center distance between individual nanorods. Releasing the load elastically relaxes the stretched polymer film, hence allowing the recording of cyclic load curves while varying the spectral response in-situ. Notably, film stretching along the substrate plane (x-axis) results in a uniaxial distortion of the nanorod lattice. We show how to account for this anisotropic strain in both the experiment and our complementary finite element modelling simulations, which then both match very well. This novel work illustrates both the feasibility and reliability when integrating 2D-GNAs for potential flexible, plasmonic applications.