Gold Nanorod Arrays Research Articles

Modelling plexcitons of periodic gold nanorod arrays with molecular components

Plasmonic or exciton/plasmon (plexcitonic) systems are presently described based on electromagnetic models, ignoring the need for an improved microscopic understanding. This is based on the fact that a full quantum mechanical approach on a micrometer scale still represents a considerable challenge. In this paper we report on the experimental observation of plexcitons in 2D gold nanorod array systems coupled to dye molecules and we provide a description of the experimental data using a quantum model. We show that treating the collective behavior in the array as being represented by a single quasiparticle is a suitable approximation that offers the opportunity to avoid the complicated calculation of long-distance interactions between the individual nanoparticles of the plexcitonic, periodic system. This enables us to model the optical response of plasmons in nanostructured arrays in contact with quantum emitters and to derive microscopic informations. Our work provides a potential tool for the design of plexcitonic devices, which rely on periodic metallic nanostructures.

Graphene oxide modification of plexciton states in the strong coupling limit

We demonstrate that gold nanorod arrays support LSPR modes which coincide with Frankel excitons in an organic J-aggregate complex forming plexciton hybrid states when tuned to within the strong coupling limit. The addition of graphene oxide modifies the strong coupling resonance conditions and Rabi frequency. This demonstrates that the formation of exciton–plasmon plexciton states in the strong coupling limit can be modified and potentially controlled through the introduction of graphene oxide which can have implications for energy harvesting or biosensor device design.

Lasing Action with Gold Nanorod Hyperbolic Metamaterials

We study lasing in two gold nanorod arrays, one exhibiting hyperbolic dispersion while the other is elliptic at the emission wavelength of Rhodamine 101. Results show the hyperbolic metamaterial provides stronger emission with reduced threshold.

Gold Nanorod Array Biochip for Label-Free, Multiplexed Biological Detection.

Gold nanorod (GNR) based label-free sensing has been attractive due to its unique property of localized surface plasmon resonance (LSPR). Compared to bulk gold, the SPR of GNRs is more sensitive to the refractive index change caused by biological binding in the close proximity. Numerous studies have reported biological detection in solution based GNR probes. However, the biosensing has the intrinsic problems of fluctuating readings and short storage time due to nanoparticle aggregation. In contrast, a chip-based nanorod biosensor is a more robust and reliable platform. We have developed a nanoplasmonic biosensor in a chip format by immobilizing functionalized GNRs on a (3-mercaptopropyl)trimethoxysilane modified glass substrate. The covalent Au-S bond ensures a strong GNR deposition on the substrate. This biochip exhibits a high sensitivity and stability when exposed to physiological buffer with high ionic strength. Another advantage of GNR as optical transducer is its LSPR peak dependence on the aspect ratio, which provides an ideal multiplexed detection mechanism. GNRs of different sizes that exhibit distinct SPR peaks are combined and deposited on designated spots of a glass substrate. The spectral shift of the respective peaks upon the biological binding are monitored for simultaneous detection of specific analytes. Coupled with a microplate reader, this spatially resolved GNR array biochip results in a high-throughput assay of samples as well as multiplexed detection in each sample. Since most biological molecules such as antibodies and DNA can be linked to GNR using previously reported surface chemistry protocol, the label-free nanosensor demonstrated here is an effective tool for protein/DNA array analysis, especially for detection of disease biomarkers.

Investigation on the Enhanced Fluorescence Emission from Self-Assembled Au Nanorod Film

A two-dimension distributed metallic nanostructure substrate containing two-dimensional array of Au nanorods with different configurations is prepared via a self-assembly method. Enhanced fluorescence emission with assembled substrate is investigated by employing Rhodamine 6G as probe molecule. The experimental observation shows that the prepared nanostructure substrate has strong fluorescence enhancement effect and the enhancement effect critically depends on the configuration of the particle distribution. The study demonstrates that the local electromagnetic field enhancement around the gold nanorod array plays an important role in achieving fluorescence enhancement with the assembled substrate array. This work suggests that the assembled substrate can be considered as a potential candidate for effective enhancement application.

Surface-Plasmon-Coupled Fluorescence Enhancement Based on Ordered Gold Nanorod Array Biochip for Ultrasensitive DNA Analysis.

An innovative gold nanorod (GNR) array biochip was developed to systematically investigate the localized surface plasmon resonance (LSPR)-coupled fluorescence enhancement for signal amplification in molecular beacon detection. An ordered GNR assembly in vertical standing array on a glass surface was fabricated as plasmonic substrates, resulting in dramatically intensified LSPR between adjacent nanoparticles as compared to that from an ensemble of random nanorods. We have shown that the plasmonic response of the nanoarray can be tuned by the proper choice of GNR size to overlap the fluorophore excitation and emission wavelengths greater than 600 nm. Plasmon-induced fluorescence enhancement was found to be distance-dependent with the competition between quenching and enhancement by the metal nanostructures. The augmented fluorescence enhancement by the GNR array can efficiently overcome the quenching effect of the gold nanoparticle even at close proximity. The enhancement correlates with the spectral overlap between the fluorophore excitation/emission and the plasmonic resonance of the GNR array, indicating a surface-plasmon-enhanced excitation and radiative mechanism for the amplification. From these results, the applicability of the ordered GNR array chip was extended to molecular fluorescence enhancement for practical use as a highly functional and ultrasensitive plasmonic DNA biochip in molecular beacon fashion.

Resonant Enhancement of Photoluminescence Intensity and Anisotropy of Quantum Dot Monolayers with Self-Assembled Gold Nanorods

Quantum dot (QD) films are well known as promising materials for photo-detectors and photovoltaic and next generation display devices. In this study, we show, experimentally, how compact monolayer films of cadmium selenide (CdSe) QDs having compact assemblies of partially aligned metal nanorods placed in close proximity can be used to enhance the photoluminescence (PL) emission intensity by greater than ten times while the PL anisotropy can be increased to almost 0.9. Finite difference time domain (FDTD) simulations on smaller scale gold nanorod (GNR) arrays on CdSe QD layer not only explain this huge emission enhancement and anisotropy observed experimentally but also provide insight into the parameters which if further optimized can lead to even larger enhancement and emission anisotropy in similar hybrid systems. Our work paves the way for creation of large scale, simple and inexpensive, but highly efficient, metal nanorod-QD hybrid films which can find wide ranging potential applications in displays, detectors, and photovoltaics.

Plasmon-enhanced Kerr nonlinearity via subwavelength-confined anisotropic Purcell factors

We theoretically investigate the enhancement of Kerr nonlinearity through anisotropic Purcell factors provided by plasmon nanostructures. In a three-level atomic system with crossing damping, larger anisotropism of Purcell factors leads to more enhanced Kerr nonlinearity in electromagnetically induced transparency windows. While for fixed anisotropic Purcell factors, Kerr nonlinearity with orthogonal dipole moments increases with the decrease of its crossing damping, and Kerr nonlinearity with nonorthogonal dipole moments is very sensitive to both the value of crossing damping and the orientation of the dipole moments. We design the non-resonant gold nanorods array, which only provides subwavelength-confined anisotropic Purcell factors, and demonstrate that the Kerr nonlinearity of cesium atoms close to the nanorods array can be modulated at the nanoscale. These findings should have potential application in ultracompact quantum logic devices.

Plasmonic mesostructures with aligned hotspots on highly oriented mesoporous silica films

Gold mesostructures are fabricated by oblique angle deposition on highly oriented mesoporous silica (MPS) thin films utlizing the periodic surface corrugation on the scale of 10 nm as a prepattern for controlled deposition. Scanning electron microscopy analysis clarified that the mesostructures comprised an array of interconnected gold nanorods oriented in plane to form meso gratings. We measured the surface enhanced Raman scattering (SERS) to demonstrate that the nanosized gaps between the rods act as hotspots. Although the sample was as thin as 8.0 nm, large SERS signals appeared because of the very narrow gaps (< 2 nm). The spatial mapping confirmed the uniform distribution of hotspots over the sample.

Multiplexed Nanoplasmonic Temporal Profiling of T-Cell Response under Immunomodulatory Agent Exposure.

Immunomodulatory drugs—agents regulating the immune response—are commonly used for treating immune system disorders and minimizing graft versus host disease in persons receiving organ transplants. At the cellular level, immunosuppressant drugs are used to inhibit pro-inflammatory or tissue-damaging responses of cells. However, few studies have so far precisely characterized the cellular-level effect of immunomodulatory treatment. The primary challenge arises due to the rapid and transient nature of T-cell immune responses to such treatment. T-cell responses involve a highly interactive network of different types of cytokines, which makes precise monitoring of drug-modulated T-cell response difficult. Here, we present a nanoplasmonic biosensing approach to quantitatively characterize cytokine secretion behaviors of T cells with a fine time-resolution (every 10 min) that are altered by an immunosuppressive drug used in the treatment of T-cell-mediated diseases. With a microfluidic platform integrating antibody-conjugated gold nanorod (AuNR) arrays, the technique enables simultaneous multi-time-point measurements of pro-inflammatory (IL-2, IFN-γ, and TNF-α) and anti-inflammatory (IL-10) cytokines secreted by T cells. The integrated nanoplasmonic biosensors achieve precise measurements with low operating sample volume (1 μL), short assay time (∼30 min), heightened sensitivity (∼20–30 pg/mL), and negligible sensor crosstalk. Data obtained from the multicytokine secretion profiles with high practicality resulting from all of these sensing capabilities provide a comprehensive picture of the time-varying cellular functional state during pharmacologic immunosuppression. The capability to monitor cellular functional response demonstrated in this study has great potential to ultimately permit personalized immunomodulatory treatment.

Gold Nanorod Arrays: Excitation of Transverse Plasmon Modes and Surface-Enhanced Raman Applications

We describe a method for the fabrication of gold nanorod arrays and their characterization. The initial requirements were for a robust substrate with strong surface enhancement of Raman scattering with minimal surface contamination. The rods with length up to 20 μm were electrochemically deposited in alumina templates bonded to gold-coated silicon wafers. After the removal of the template, surface-bound cyanide from the deposition process was removed by mild electropolishing. Reflectance and cathodoluminescence measurements revealed transverse dipole and quadrupole plasmon modes. Surface enhancement of 106 was determined by taking Raman spectra of self-assembled monolayers of 4-aminothiophenol. The enhancement factor (EF) was found to be independent of nanorod aspect ratio because only the length of the nanorods was varied while their diameter was kept constant, and because of the large length of nanorods only the transverse plasmons were excited.

Tunable Lattice Coupling of Multipole Plasmon Modes and Near-Field Enhancement in Closely Spaced Gold Nanorod Arrays.

Considering the nanogap and lattice effects, there is an attractive structure in plasmonics: closely spaced metallic nanoarrays. In this work, we demonstrate experimentally and theoretically the lattice coupling of multipole plasmon modes for closely spaced gold nanorod arrays, offering a new insight into the higher order cavity modes coupled with each other in the lattice. The resonances can be greatly tuned by changes in inter-rod gaps and nanorod heights while the influence of the nanorod diameter is relatively insignificant. Experimentally, pronounced suppressions of the reflectance are observed. Meanwhile, the near-field enhancement can be further enhanced, as demonstrated through surface enhanced Raman scattering (SERS). We then confirm the correlation between the near-field and far-field plasmonic responses, which is significantly important for maximizing the near-field enhancement at a specific excitation wavelength. This lattice coupling of multipole plasmon modes is of broad interest not only for SERS but also for other plasmonic applications, such as subwavelength imaging or metamaterials.

Ultrasensitive Characterization of Mechanical Oscillations and Plasmon Energy Shift in Gold Nanorods.

Mechanical vibrational resonances in metal nanoparticles are intensively studied because they provide insight into nanoscale elasticity and for their potential application to ultrasensitive mass detection. In this paper, we use broadband femtosecond pump-probe spectroscopy to study the longitudinal acoustic phonons of arrays of gold nanorods with different aspect ratios, fabricated by electron beam lithography with very high size uniformity. We follow in real time the impulsively excited extensional oscillations of the nanorods by measuring the transient shift of the localized surface plasmon band. Broadband and high-sensitivity detection of the time-dependent extinction spectra enables one to develop a model that quantitatively describes the periodic variation of the plasmon extinction coefficient starting from the steady-state spectrum with only one additional free parameter. This model allows us to retrieve the time-dependent elongation of the nanorods with an ultrahigh sensitivity and to measure oscillation amplitudes of just a few picometers and plasmon energy shifts on the order of 10(-2) meV.

Preparation and Biosensing Performance of Porous-alumina-Assisted Gold Nanostructures on Substrates

Gold based nanosize structures such as nanoparticles and nanowires are considered as important building blocks for nanotechnology to be used for construction of sensing micro- and nanodevices because of their excellent optical, electrical and mechanical properties. Here we synthesize via porous-alumina-assisted anodizing and electrodeposition perfect arrays of gold nanodots and nanorods, upright standing, spatially separated and highly aligned on a conductive substrate, which are expected to have exceptional performance in nanoscale electronic, optoelectronics and sensing applications. Biosensing potential of the nanostructured electrodes was assessed by examining electrochemical properties of a protein layer grafted on the electrode for C-reactive protein (CRP) detection using CRP-antibody immobilization method with protein G intermediate layer. The laboratory prototype of immunosensor revealed reproducible, rapid and stable response with a low limit for CRP-antigen detection of 100 fg·ml-1

Realizing Broadband and Invertible Linear-to-circular Polarization Converter with Ultrathin Single-layer Metasurface.

The arbitrary control of the polarization states of light has attracted the interest of the scientific community because of the wide range of modern optical applications that such control can afford. However, conventional polarization control setups are bulky and very often operate only within a narrow wavelength range, thereby resisting optical system miniaturization and integration. Here, we present the basic theory, simulated demonstration, and in-depth analysis of a high-performance broadband and invertible linear-to-circular (LTC) polarization converter composed of a single-layer gold nanorod array with a total thickness of ~λ/70 for the near-infrared regime. This setup can transform a circularly polarized wave into a linearly polarized one or a linearly polarized wave with a wavelength-dependent electric field polarization angle into a circularly polarized one in the transmission mode. The broadband and invertible LTC polarization conversion can be attributed to the tailoring of the light interference at the subwavelength scale via the induction of the anisotropic optical resonance mode. This ultrathin single-layer metasurface relaxes the high-precision requirements of the structure parameters in general metasurfaces while retaining the polarization conversion performance. Our findings open up intriguing possibilities towards the realization of novel integrated metasurface-based photonics devices for polarization manipulation, modulation, and phase retardation.

Portable kit for identification and detection of drugs in human urine using surface-enhanced Raman spectroscopy.

A portable kit was demonstrated for rapid and reliable surface-enhanced Raman scattering (SERS) detection of drugs in human urine. This kit contains two sealed reagent tubes, a packet of standardized SERS substrates, and a mini Raman device. A 3 min pretreatment for separating amphetamines from human urine was developed with an extraction rate of >80% examined by ultraperformance liquid chromatography (UPLC). Simultaneously, highly reproducible two-dimensional (2D) gold nanorod (GNR) arrays were assembled by the use of methoxymercaptopoly(ethylene glycol) (mPEG-SH) capping. Thirty batches of GNR arrays produced the 1001 cm(-1) intensity of methamphetamine (MA) molecules with a relative standard deviation (RSD) of 7.9%, and a 21 × 21 μm(2) area mapping on a 2D GNR array produced a statistical RSD of <10%, implying an excellent reproducibility and uniformity. The detection limit of amphetamines in human urine was at least 0.1 ppm. Moreover, the portable kit was successfully used for detecting MA, 3,4-methylenedioxymethamphetamine (MDMA), and methcathinone (MC) in 30 volunteers' urine samples with various clinical natures, and the dual-analyte detection of MA and MDMA implied a good capability of multiplex analysis. UPLC examination and the SERS recovery test clearly indicated that our pretreatment procedure was sufficient to lower the high background signals caused by complex components in urine and demonstrated the practicability and the resistance to false positives, which is a vital problem for law enforcement applications. The excellent performance of our portable kit promises a great prospective toward a rapid, reliable, and on-spot analyzer, especially for public safety and healthcare.

Preparation of Gold Nanorods Using 1,2,4-Trihydroxybenzene as a Reducing Agent.

We report an improved method for synthesizing gold nanorods (GNRs) by using 1,2,4-trihydroxybenzene as a reducing agent. The method allows a rich array of monodispersed GNRs with longitudinal surface plasmon resonance (LSPR) tunable from 698 to 913 nm to be generated. A large range of diameter distribution of GNRs from 9.3 to 32.2 nm with exceptional monodispersity can be well prepared by this method. These findings indicate that this method has greater performance in controlling the morphology of GNRs than that of traditional approaches with ascorbic acid as a reductant.

Vertically aligned self-assembled gold nanorods as low turn-on, stable field emitters

In this work we have investigated field emission from self-assembled, vertically aligned, gold nanorod arrays, which were synthesized via a colloidal growth method. A field emission current density of ∼1mA/cm2 was measured for these gold nanorod arrays using an anode-cathode separation of ∼3.5mm. The field emission investigation of these gold nanorod arrays was carried out at a base pressure of ∼10−8mbar. The turn on field, defined as the electric field required to obtain a current density of 1μA/cm2, is observed to be 1.9V/μm. Assuming a work function value of 5.3eV, the field enhancement factor β is estimated to be ∼2931, which is higher than the reported values for other gold nanostructures/arrays.

Light‐Driven Unidirectional Liquid Motion on Anisotropic Gold Nanorod Arrays

A facile concept by combining plasmonic directional gold nanorod arrays with a temperature-sensitive polymeric thin film for controlling water motion through light illumination is demonstrated. The developed concept does not only allow the flow and guidance of water through light illumination, but also allows the flow rate and direction of the liquid to be easily controlled.

Plasmon enhanced fluorescence studies from aligned gold nanorod arrays modified with SiO2 spacer layers

Here, we demonstrate that quasi self-standing Au nanorod arrays prepared with plasma polymerisation deposited SiO2 dielectric spacers support surface enhanced fluorescence (SEF) while maintaining high signal reproducibility. We show that it is possible to find a balance between enhanced radiative and non-radiative decay rates at which the fluorescent intensity is maximized. The SEF signal optimised with a 30 nm spacer layer thickness showed a 3.5-fold enhancement with a signal variance of &amp;lt;15% thereby keeping the integrity of the nanorod array. We also demonstrate the decreased importance of obtaining resonance conditions when localized surface plasmon resonance is positioned within the spectral region of Au interband transitions. Procedures for further increasing the SEF enhancement factor are also discussed.