Gold Arrays Research Articles

Novel 3D arrays of gold nanostructures on suspended platinum-coated carbon nanotubes as surface-enhanced Raman scattering substrates

We report electrochemically controlled fabrications and the corresponding enhancement in Raman scattering of the three-dimensional (3D) gold arrays prepared on the template of platinum-coated carbon nanotubes (CNTs) by electrochemical deposition (ECD). The topography of deposited gold was varied from nanoparticles to nanowires by ECD conditions. The increasing surface area of 3D arrays of gold nanoparticles contributed to increase significantly the sensitivity of surface enhanced Raman scattering (SERS). For the same 3D arrays consisting of gold nanowires but with different surface roughness, much higher SERS activity was obtained when the surface of nanowires was rough.

Gold mesoflower arrays with sub-10 nm intraparticle gaps for highly sensitive and repeatable surface enhanced Raman spectroscopy

Self-assembling Au mesoflower arrays are prepared using a polymethylmethacrylate (PMMA) template on an iron substrate via a combined top-down/bottom-up nanofabrication strategy. The PMMA template with the holes around 300–500 nm in diameter is first fabricated by using polymer blend lithography on iron substrates, and the highly homogeneous Au mesoflower arrays with less than 10 nm intraparticle gaps are subsequently obtained by an in situ galvanic reaction between HAuCl4 solution and the iron substrate under optimal stirring of the solution as well as reaction time. Owing to the unique mesostructures and uniformity, Raman measurements show that the gold mesoflower arrays obtained demonstrated a strong and reproducible surface enhanced Raman scattering (SERS) enhancement on the order of ∼107–108. The development of a SERS substrate based on the Au mesoflowers with high spatial density of hot spots, relatively low cost and facial synthesis provides a novel strategy for applications in chemical and biomolecular sensing.

Inkjet Printing of Nanoporous Gold Electrode Arrays on Cellulose Membranes for High-Sensitive Paper-Like Electrochemical Oxygen Sensors Using Ionic Liquid Electrolytes

A simple approach to the mass production of nanoporous gold electrode arrays on cellulose membranes for electrochemical sensing of oxygen using ionic liquid (IL) electrolytes was established. The approach, combining the inkjet printing of gold nanoparticle (GNP) patterns with the self-catalytic growth of these patterns into conducting layers, can fabricate hundreds of self-designed gold arrays on cellulose membranes within several hours using an inexpensive inkjet printer. The resulting paper-based gold electrode arrays (PGEAs) had several unique properties as thin-film sensor platforms, including good conductivity, excellent flexibility, high integration, and low cost. The porous nature of PGEAs also allowed the addition of electrolytes from the back cellulose membrane side and controllably produced large three-phase electrolyte/electrode/gas interfaces at the front electrode side. A novel paper-based solid-state electrochemical oxygen (O(2)) sensor was therefore developed using an IL electrolyte, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF(6)). The sensor looked like a piece of paper but possessed high sensitivity for O(2) in a linear range from 0.054 to 0.177 v/v %, along with a low detection limit of 0.0075% and a short response time of less than 10 s, foreseeing its promising applications in developing cost-effective and environment-friendly paper-based electrochemical gas sensors.

Resonantly enhanced optical nonlinearity in hybrid semiconductor quantum dot – metal nanoparticle structures

The optical nonlinearity of hybrid structures composed of CdTe quantum dots and periodical particle array of gold is studied using Z-scan method. The optical nonlinearity is dramatically affected by the interaction between exciton in CdTe quantum dots and surface plasmons in Au periodical particle array. When the Au surface plasmon is tuned to be in resonance with the exciton transition in CdTe quantum dots, the largest nonlinear refractive index and the smallest two-photon absorption coefficient, n2 = −0.53 cm2/GW and β = 25 cm/GW, which are about 8 times larger and 50 times smaller than that of bare CdTe quantum dots, can be achieved.

Preparation and Characterization of Electrodeposited ZnO on Microstructured Electrode Arrays

Interdigitated microstructured electrode arrays of gold or silver bands were used as substrates for the electrodeposition of crystalline ZnO thin films under pulsed galvanostatic conditions from aqueous Zn(NO3)2 solutions. For current densities below the limiting current density, homogenous films of ZnO were obtained as proven by scanning electron and confocal laser microscopy. For current densities greater than those limited by the diffusion of Zn2+ and NO3−, strongly dendritic structures were formed. Film growth was analyzed based on the duplex diffusion layer model. The influence of convective diffusion was directly observed when only one set of electrode bands was polarized during deposition. By use of an underlying gate electrode in the substrate, field-effect measurements were performed to electrically characterize the samples.

Gold patterned biochips for on-chip immuno-MALDI-TOF MS: SPR imaging coupled multi-proteinMS analysis

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis of immuno-captured target protein efficiently complements conventional immunoassays by offering rich molecular information such as protein isoforms or modifications. Direct immobilization of antibodies on MALDI solid support enables both target enrichment and MS analysis on the same plate, allowing simplified and potentially multiplexing protein MS analysis. Reliable on-chip immuno-MALDI-TOF MS for multiple biomarkers requires successful adaptation of antibody array biochips, which also must accommodate consistent reaction conditions on antibody arrays during immuno-capture and MS analysis. Here we developed a facile fabrication process of versatile antibody array biochips for reliable on-chip MALDI-TOF-MS analysis of multiple immuno-captured proteins. Hydrophilic gold arrays surrounded by super-hydrophobic surfaces were formed on a gold patterned biochip via spontaneous chemical or protein layer deposition. From antibody immobilization to MALDI matrix treatment, this hydrophilic/phobic pattern allowed highly consistent surface reactions on each gold spot. Various antibodies were immobilized on these gold spots both by covalent coupling or protein G binding. Four different protein markers were successfully analyzed on the present immuno-MALDI biochip from complex protein mixtures including serum samples. Tryptic digests of captured PSA protein were also effectively detected by on-chip MALDI-TOF-MS. Moreover, the present MALDI biochip can be directly applied to the SPR imaging system, by which antibody and subsequent antigen immobilization were successfully monitored.

Fabrication of immunosensor microwell arrays from gold compact discs for detection of cancer biomarker proteins.

A simple method is reported to fabricate gold arrays featuring microwells surrounding 8-electrodes from gold compact discs (CDs) for less than $0.2 per chip. Integration of these disposable gold CD array chips with microfluidics provided inexpensive immunoarrays that were used to measure a cancer biomarker protein quickly at high sensitivity. The gold CD sensor arrays were fabricated using thermal transfer of laserjet toner from a computer-printed pattern followed by selective chemical etching. Sensor elements had an electrochemically addressable surface area of 0.42 mm(2) with RSD <2%. For a proof-of-concept application, the arrays were integrated into a simple microfluidic device for electrochemical detection of cancer biomarker interleukin-6 (IL-6) in diluted serum. Capture antibodies of IL-6 were chemically linked onto the electrode arrays and a sandwich immunoassay protocol was developed. A biotinylated detection antibody with polymerized horseradish peroxidase labels was used for signal amplification. The detection limit of IL-6 in diluted serum was remarkably low at 10 fg mL(-1) (385 aM) with a linear response with log of IL-6 concentration from 10 to 1300 fg mL(-1). These easily fabricated, ultrasensitive, microfluidic immunosensors should be readily adapted for sensitive detection of multiple biomarkers for cancer diagnostics.

Terahertz metamaterials based on arrays of rolled-up gold/(In)GaAs tubes

We investigate metamaterials based on arrays of rolled-up gold/(In)GaAs microrolls. By finite-integration-technique simulations we show that these arrays interact resonantly with the magnetic component of an electromagnetic field and exhibit a negative effective permeability at terahertz frequencies. We find a strong dependence of the resonance frequency on small variations in the winding number n. We show that this dependence can be removed, if desired, by applying an additional slit into the metal layer of the tube.

Two-Dimensional Arrays of Gold Nanoparticles for Plasmonic Nanosensor

Two dimensional (2D) arrays of noble metal nanoparticles are widely used in the sensing of nanoscale biological and chemical events. Research in this area has sparked considerable interest in many fields owing to the novel optical properties, e.g., the localized surface plasmon resonance, of these metallic nanoarrays. In this paper, we report successes in fabricating 2D arrays of gold nano-islands using nanosphere lithography. The reproducibility and the effectiveness of the nano-patterning method are tested by means of spin coating and capillary force deposition. We found that the capillary force deposition method was more effective for nanospheres with diameters greater than 600 nm, whereas the spin coating method works better for nanospheres with diameters less than 600 nm. The optimal deposition parameters for both methods were reported, showing about 80% reproducibility. In addition, we characterize gold nano-island arrays both geometrically with AFM as well as optically with UV-VIS spectrometry. The AFM images revealed that the obtained nano-arrays formed a hexagonal pattern of truncated tetrahedron nano-islands. The experimental and theoretical values of the geometric parameters were compared. The 2D gold nano-arrays showed strong LSPR in the absorption spectra. As the nano-islands increased in size, the LSPR absorption bands became red-shifted. Linear dependence of the plasmon absorption maximum on the size of the gold nano-islands was identified through the increment in the plasmon absorption maximum rate for a one nanometer increase in the characteristic length of the nano-islands. We found that the 2D gold nano-arrays showed nearly seven-fold higher sensitivity of the absorption spectrum to the size of the nano-islands as compared to colloidal gold nano-particles.

Investigation of the nonlinear optical properties of metamaterials by second harmonic generation

Nonlinear optical spectroscopy is applied to two types of metamaterials: (i) variations of split-ring resonators fabricated as periodic two-dimensional arrays of gold on SiO2, and (ii) gratings of periodically spaced straight wires of gold on multiferroic hexagonal single crystals from the RMnO3 family. Optical second harmonic generation (SHG) on samples of type (i) is used to characterize the nonlinear response of metamaterials in dependence of the photon energy and the size, shape, and symmetry of the nanoscopic building blocks constituting the metamaterials. We find a complex dependence of the SHG spectra on the design parameters that requires an in-depth theoretical analysis. SHG is applied to samples of type (ii) to enhance the nonlinear optical performance of functional materials by writing a metamaterial acting as optical catalyst on top. We observe an enhancement of the SHG signal induced by the ferroelectric order of the RMnO3 crystals, yet with pronounced variations between samples. Measurements of both types, (i) and (ii), show that a comprehensive understanding of the nonlinear optical properties of metamaterials requires one to go beyond a macroscopic symmetry treatment. In fact, structural inhomogeneities on the level of the nanoscopic building blocks have a major impact.

A label-free optical sensor based on nanoporous gold arrays for the detection of oligodeoxynucleotides

Interest in using nanoporous materials for sensing applications has increased. The present study reports a method of preparing well-ordered nanoporous gold arrays using a porous silicon (PSi) template. Gold nanolayer could be electrodeposited on the surface of the PSi template at low electrolysis currents in low concentration of chloroauric acid (HAuCl 4) solution. Surface morphology characterizations and optical measurements revealed that a PSi-templated nanoporous gold (Au–PSi) array well replicated the nanoporous structure and retained the optical properties of PSi. Fourier transform reflectometric interference spectra showed that a characteristic blue-shifted effective optical thickness (EOT) was observed due to the low refractive index of the gold film. An optical DNA biosensor was then fabricated via the self-assembly of single-stranded DNA (ssDNA) with a specific sequence on the surface of Au–PSi. The attachment of ssDNA and its hybridization with target oligonucleotides (ODNs) persistently caused the blue shift of the EOT. Consequently, a relationship between the EOT shift and the ODN concentration was established. The mechanism of the optical response caused by DNA hybridization on the Au–PSi surface was qualitatively explained by the electromagnetic theory and electrochemical impedance spectroscopy (EIS). The lowest detection limit for target ODNs was estimated at around 10 −14 mol L −1, when the baseline noise, a variation in the value of EOT is around 5 nm. The fabricated Au–PSi based optical biosensor has potential use in the discovery of new ODN drugs because it will be able to detect the binding event between ODNs and the target DNA.

Investigation of plasmonics resonance infrared bowtie metal antenna

An approximate resonance wavelength equation that varies with metal antenna structure size is developed to design a bowtie gold metal antenna working at near-infrared (IR) wavelength. Bowtie antenna structures with resonance wavelength of 1.06 μm, 1.55 μm and 10.6 μm are designed based on this equation. A finite-difference time domain (FDTD) algorithm with total field scattered field (TFSF) source simulation shows the resonance wavelength of the designed structures being precisely in agreement with the expected wavelengths from the equation. Planar integration of the metal bowtie antennas is discussed as well. Gold nanohole bowtie antenna arrays are fabricated and the near-field optical transmission properties of the nanohole array are investigated with a near-field scanning optical microscope (NSOM). Our experimental results verify the near-field optical transmission performance and further demonstrate that they are in agreement with the theoretical calculation results. The high enhancement efficiency and integration of the metal bowtie antennas open the possibility of a wide application in IR optoelectronics detection and imaging.

Solvent induced formation of an ordered nanorod array of gold/polymer composite by blockcopolymer film templating

We report a novel method to fabricate ordered arrays of gold–polymer composite nanorodswith the orientation in the vertical direction using block copolymer (BCP) film.The salt precursor is selectively infiltrated within vertically aligned cylindricaldomains of the BCP film by immersing the template in a simple aqueous solution ofHAuCl4. Scanning electron microscopy suggests that the salt might be uniformly positioned along thepolymeric cylinders. A subsequent vacuum ultraviolet light irradiation simultaneously reduces theHAuCl4 into spherical gold nanoparticles with mean diameter around 2 nm and removes the matrixof the BCP template to produce metal–polymer composite nanorods. While the solvent ismethanol, the salt might be concentrated at the bottom of the BCP film. As a result, aperiodic pattern of gold nanoparticles with average diameter around 11 nm is formed wherethe BCP film is completely etched away. The solvent can effectively tune the spatialdistribution of the salt precursor along the polymeric cylinders, which is responsiblefor the different morphologies of the photochemically fabricated nanostructures.

From Single to Multiple Ag-Layer Modification of Au Nanocavity Substrates: A Tunable Probe of the Chemical Surface-Enhanced Raman Scattering Mechanism

We present experimental and computational results that enlighten the mechanisms underlying the chemical contribution to surface-enhanced Raman scattering (SERS). Gold void metallic arrays electrochemically covered either by a Ag monolayer or 10-100 Ag layers were modified with a self-assembled monolayer of 4-mercaptopyridine as a molecular Raman probe displaying a rich and unexpected Raman response. A resonant increase of the Raman intensity in the red part of the spectrum is observed that cannot be related to plasmon excitations of the cavity-array. Notably, we find an additional 10-20 time increase of the SERS amplification upon deposition of a single Ag layer on the Au substrate, which is, however, almost quenched upon deposition of 10 atomic layers. Further deposition of 100 atomic Ag layers results in a new increase of the SERS signal, consistent with the improved plasmonic efficiency of Ag bulk-like structures. The SERS response as a function of the Ag layer thickness is analyzed in terms of ab initio calculations and a microscopic model for the SERS chemical mechanism based on a resonant charge transfer process between the molecular HOMO state and the Fermi level in the metal surface. We find that a rearrangement of the electronic charge density related to the presence of the Ag monolayer in the Au/Ag/molecule complex causes an increase in the distance between the HOMO center of charge and the metallic image plane that is responsible for the variation of Raman enhancement between the studied substrates. Our results provide a general platform for studying the chemical contribution to SERS, and for enhancing the Raman efficiency of tailored Au-SERS templates through electrochemical modification with Ag films.

Template synthesis of nanoparticle arrays of gold using highly ordered functional mesoporous silica thin films with high amino-groups content

The periodic disposed gold nanoparticle arrays (GNPAs) are very useful for wide range applications. Employing the neutralization reaction between chloroauric acid and amino groups, GNPAs were self-assembled inside pore channels of amino-functionalized mesoporous silica thin films (AF-MSTFs) which were produced by using the surface active agent CTAB (cetyltrimethyl ammonium bromide). The AF-MSTF samples were characterized by FTIR, XRD and TEM. The results show that the average diameter of quantum dots was controlled by the highly ordered pore channels. UV–vis absorption spectra of the GNPAs/AF-MSTF nanocomposites exhibit blue-shift and intensity decrease of absorption peaks as the neutralization reaction time decreases, representing the existence of GNPs and revealing an adjustable optical property.

A realtime and continuous assessment of cortisol in ISF using electrochemical impedance spectroscopy

This study describes the functioning of a novel sensor to measure cortisol concentration in the interstitial fluid (ISF) of a human subject. ISF is extracted by means of vacuum pressure from micropores created on the stratum corneum layer of the skin. The pores are produced by focusing a near infrared laser on a layer of black dye material attached to the skin. The pores are viable for approximately three days after skin poration. Cortisol measurements are based on electrochemical impedance (EIS) technique. Gold microelectrode arrays functionalized with Dithiobis (succinimidyl propionate) self-assembled monolayer (SAM) have been used to fabricate an ultrasensitive, disposable, electrochemical cortisol immunosensor. The biosensor was successfully used for in vitro measurement of cortisol in ISF. Tests in a laboratory setup show that the sensor exhibits a linear response to cortisol concentrations in the range 1pM to 100nM. A small pilot clinical study showed that in vitro immunosensor readings, when compared with commercial evaluation using enzyme-linked immunoassay (ELISA) method, correlated well with cortisol levels in saliva and ISF. Further, circadian rhythm could be established between the subject's ISF and the saliva samples collected over 24h time-period. Cortisol levels in ISF were found reliably higher than in saliva. This research establishes the feasibility of using impedance based biosensor architecture for a disposable, wearable cortisol detector. The projected commercial in vivo real-time cortisol sensor device, besides being minimally invasive, will allow continuous ISF harvesting and cortisol monitoring over 24h even when the subject is asleep. Forthcoming, this sensor could be interfaced to a wireless health monitoring system that could transfer sensor data over existing wide-area networks such as the internet and a cellular phone network to enable real-time remote monitoring of subjects.

Optical and electrochemical properties of ordered macroporous gold array on the ITO surface

The electrochemical and optical properties of transparent, two-dimensional macroporous gold film were investigated. Colloidal crystal templates were assembled onto indium-doped tin oxide (ITO) glass surface through vertical depositing method. Following gold electrodeposition, they were removed by dissolution with tetrahydrofuran (THF). The highly ordered macroporous gold array was achieved. It was characterized by scanning electron microscope (SEM) and ultraviolet visible (UV–vis) spectrophotometry. The optical transparency of the gold film was near 25% and fairly constant between 300 and 900 nm. The macroporous gold film electrode was mounted into a thin-layer transmission cell. The electrochemical response was evaluated by thin-layer cyclic voltammograms (CV) of the Fe (CN) 6 3−/Fe (CN) 6 4− couple. Thin-layer cell exhibits good shape of waves and nearly symmetric cathodic and anodic waves. E 0′ value and n of TMPD + /TMPD (TMPD is acronyms for N,N,N′,N′-tetramethyl-p-phenylenediamine, and TMPD + is its mono-cation radical) couple were determined. Furthermore, results demonstrated electrolytic equilibrium was faster reached in macroporous gold film than ITO electrode.

Towards individual electrical contact of nanoparticles in nanocomposites

A new technological approach that will allow contacting individual nano-objects is presented. An ultra-thin nanocomposite layer is formed by embedding semiconductor and metal nanoparticles in a highly insulating epoxy based negative photoresist. The role of the photoresist is to support and electrically insulate the nanoparticles. In order to perform electrical contacts, an array of gold micro/nano-electrodes has been defined by electron beam lithography on top of the nanocomposite layer. Promising initial results of the electrical characterization are presented, evidencing significant differences between bare resist and nanocomposite films.

Regio-selective decoration of nanocavity metal arrays: contributions from localized and delocalized plasmons to surface enhanced Raman spectroscopy

Spherical cap gold nanocavity arrays with internal diameters of 240, 430, 600 and 820 nm were fabricated on smooth gold films using nanosphere lithography with electrochemical metal deposition. Each array was prepared to the same normalized film thickness to diameter ratios, t(N), of 0.8 ± 0.04. Selective modification of the top surface and interior walls of the gold nanocavity arrays with [Ru(bpy)(2)(Qbpy)](2+), where bpy is 2,2'-bipyridyl and Qbpy is 2,2':4,4'':4,4''-quarterpyridyl, was accomplished using a two step adsorption process exploiting the assembled polystyrene spheres as masks. This selective modification approach permitted direct quantitative comparison, for the first time, of plasmonic enhancement of Raman signal and luminescence signal from a monolayer adsorbed at the top surface versus interior walls of all-gold nanocavity arrays. For all cavity sizes, significantly greater Raman and luminescence signal enhancement was observed from [Ru(bpy)(2)(Qbpy)](2+) monolayer adsorbed at the top surface of the array compared with the cavity walls. This disparity in Raman intensity from top versus cavity interior increased as the cavity dimensions decreased. For example, the Raman signal intensity from [Ru(bpy)(2)(Qbpy)](2+) adsorbed at the top surface of 240 nm gold arrays was 170 times greater than SERS signal for this material adsorbed at the interior walls of this array, whereas the relative Raman signal enhancement was 6 from top versus interior for the 820 nm internal radius arrays under 785 nm excitation. The origin of the relatively greater signal at the top surface is discussed in the context of plasmonic distribution at each surface.

Fabrication of gold sphere to cuboid nanoarrays using PDMS templates

A novel polymer based templated approach to creating highly ordered gold arrays with either positive or negative nano-scale features of controlled shape from truncated spheres to cuboids is presented.