Gold Oxide Layer Research Articles

Square & H metasurfaces for SPR Increasing in long Wave-IR absorber

In the long-wave infrared (LWIR) spectrum, this paper suggests an electromagnetic (EM) waveband absorber design based on metamaterials. Germanium, gold, and magnesium oxide layers are arranged in a layered structure from top to bottom in the suggested model. Our proposed metamaterial structure’s upper surface is composed of metallic metasurfaces with H and square forms from various studies. The finite element method is used to evaluate the metamaterials’ electromagnetic properties in terms of absorbance and reflectance. It is observed that there is a particular size of the metamaterial at which extremely localized electromagnetic resonance occurs. Quantitative findings indicate that the suggested metamaterial design’s average absorption reaches 90 % in the 10 μm to 14 μm range across a wide variety of incidence angles (00 to 400 & 00 to 800) for both transverse electric (TE) and transverse magnetic (TM) polarization. It is evident from these data that the suggested model configuration has broad potential applications in manyoptoelectronic fields of study.

Reflected light microscopy of a gold oxide layer formed on a Au film by ultraviolet/ozone treatment

The surface oxidation of Au thin films in UV/ozone and the reduction of AuOx surfaces at room temperature was investigated by X-ray photoelectron spectroscopy, work function measurements, and optical reflectance spectroscopy. The optical reflectance from the surface of an 11-nm-thick Au film formed on a Si substrate with a 120-nm-thick SiO2 layer exhibited a value close to zero at a specific wavelength around 535 nm as a result of interference in the multilayer structure of the substrate. The formation of AuOx on top of the Au slightly shifts the wavelength of the minimum reflectance. A pattern of oxidized AuOx regions and non-oxidized Au regions was formed on the surface of a sample by using a shadow mask during the UV/ozone exposure; illuminating this sample with quasi-monochromatic light in an optical microscope equipped with a digital camera enabled direct observation of the change in the reflectance caused by the formation of AuOx. The contrast of the AuOx region gradually decreased with time. The relationship between the contrast decrease and the reduction of AuOx was investigated.

A D-shaped elliptical hollow core fiber SPR sensor

A D-shaped elliptical hollow core fiber surface plasmon resonance (SPR) sensor is proposed and analyzed here using gold and titanium oxide (TiO2) layer. The fiber geometry consists of a doped elliptical core with a concentric central elliptical air hole. A finite element method-based simulation software COMSOL Multiphysics is used for numerical analysis. Both gold and TiO2 layers are optimized. The sensor responds over a wide range of refractive index (RI) from 1.33 to 1.40. The average sensitivity obtained for the designed sensor is 4142.85nm/RIU (RI Unit) whereas that for a single mode fiber (SMF) is only 897.55nm/RIU. So, our designed sensor gives more than 4 times enhanced sensitivity and proves to be a potential alternative of SMF based low cost, compatible SPR sensors by covering the low sensitivity issue suffered by SMF sensors.

Highly selective SPR based fiber optic sensor for the detection of hydrogen peroxide

The present study reports the fabrication and characterization of a surface plasmon resonance (SPR) based fiber optic hydrogen peroxide sensor using enzymatic approach. The sensing probe is fabricated by depositing gold and graphene oxide (GO) layers over the unclad core of an optical fiber followed by the immobilization of catalase enzyme via EDC-NHS coupling. In this probe, gold provides stability and repeatability to the sensor because it does not oxidize in the presence of oxygen environment like silver, GO being of high refractive index enhances the sensitivity of the sensor while the catalase provides specificity to the sensor due to its very specific interaction with hydrogen peroxide. Further, GO also plays an important role in sensing mechanism by interacting with water molecules released from the enzymatic reaction which changes its effective refractive index. GO is synthesized using modified Hummer’s method and characterized by SEM, TEM and AFM. The operation of the fabricated probe is tested for the concentration of hydrogen peroxide ranging from 0–1000 μM. The sensor possesses 55 μM as the limit of detection and its performance is pH dependent. It is highly selective, stable, repeatable and fast in response. The probe being fabricated on optical fiber substrate gives additional advantages of simplicity, miniaturization, online monitoring and remote sensing of hydrogen peroxide. These advantages promise its applications in health monitoring.

Reaction Monitoring of Gold Oxides Prepared by an Oxygen-dc Glow Discharge from Gold Films in Various Aqueous Solutions by a Surface Plasmon Resonance-based Optical Waveguide Sensing System and X-ray Photoelectron Spectroscopy.

The chemical properties of thin (∼0.6 nm) gold oxide layers prepared by an oxygen-dc glow discharge from gold films at room temperature in various solvents and solutions were studied using a surface plasmon resonance (SPR)-based optical waveguide sensing system and X-ray photoelectron spectroscopy (XPS). New insights into the stability and reactivity of gold oxides under these conditions were obtained. The O 1s XPS spectra show three oxygen species, comprising components I, II, and III in the gold oxides, and components I and II are present in this order from the top surface of the oxide (component III). The gold oxide is stable in various solvents (water, methanol, ethanol, acetone, acetonitrile, diethyl ether, chloroform, hydrocarbons) for 10 - 30 min at room temperature. The gold oxide decomposes in aqueous 10-2 M solutions of acetaldehyde, hydrochloric acid, and sodium hydroxide during these periods. Gold oxide decomposition in these solutions was monitored by SPR and the decomposition rates were obtained. Decomposition of the gold oxides was confirmed and their surfaces were characterized after decomposition by XPS.

Comparison of Argon and Oxygen Plasma Treatments for Ambient Room-Temperature Wafer-Scale Au⁻Au Bonding Using Ultrathin Au Films.

Au–Au surface activated bonding is promising for room-temperature bonding. The use of Ar plasma vs. O2 plasma for pretreatment was investigated for room-temperature wafer-scale Au–Au bonding using ultrathin Au films (<50 nm) in ambient air. The main difference between Ar plasma and O2 plasma is their surface activation mechanism: physical etching and chemical reaction, respectively. Destructive razor blade testing revealed that the bonding strength of samples obtained using Ar plasma treatment was higher than the strength of bulk Si (surface energy of bulk Si: 2.5 J/m2), while that of samples obtained using O2 plasma treatment was low (surface energy: 0.1–0.2 J/m2). X-ray photoelectron spectroscopy analysis revealed that a gold oxide (Au2O3) layer readily formed with O2 plasma treatment, and this layer impeded Au–Au bonding. Thermal desorption spectroscopy analysis revealed that Au2O3 thermally desorbed around 110 °C. Annealing of O2 plasma-treated samples up to 150 °C before bonding increased the bonding strength from 0.1 to 2.5 J/m2 due to Au2O3 decomposition.

(Invited) Electrochemical Studies of Zinc/L-Cysteine Interactions in Aqueous Buffers

The electrochemical behavior of L-cysteine has been investigated in pH 11.0 phosphate buffer at both gold and glassy carbon. Cyclic voltammetric responses show similar behavior to that in pH 7.4 phosphate and pH 4.7 acetate buffers. There is a positive 60 mV L-cysteine peak potential shift with decreasing pH, consistent with a one-electron/one-proton L-cysteine oxidation mechanism. Electrochemical impedance spectroscopic (EIS) studies of L-cysteine suggested involvement of significant surface interactions between gold and L-cysteine and/or its oxidation products. Studies of glutathione, a tripeptide incorporating L-cysteine, showed an oxidation process for the L-cysteine moiety, although little interaction with the gold oxide layer was observed by cyclic voltammetry. In a pH 4.7 acetate buffer, only minimal voltammetric changes at gold were noted upon addition of ZnSO4 to L-cysteine, showing that L-cysteine is not extensively complexed by Zn2+ at pH 4.7.

Analysis of water in room temperature ionic liquids by linear sweep, differential pulse and square wave cathodic stripping voltammetries

In this work we put forward an optimized electroanalytical method for the determination of water in room temperature ionic liquids (RTILs) based on the reduction of a gold oxide layer prepared by chronoamperometry on gold electrodes under the presence of trace water. Linear sweep, differential pulse and square wave voltammetries were compared as stripping techniques, and the chronoamperometric growth of the gold oxide layer was studied and optimized, confirming the formation of up to three monolayers of AuO on the surface of the working electrode. For [C2mim][NTf2], dried for 12h at 60°C, concentrations of water of 110.5±4.6ppm, a sensitivity of −ip,c/C=(10.9±0.4)×10−7Appm−1, a LOD=3.3ppm and a LOQ=11.0ppm were found using the proposed CSSWV method.

Gold oxide films grown in the confined aqueous layer between gold and organic solvents

The properties of anodic passive films potentiostatically formed on polycrystalline gold in aqueous phosphate solutions were studied using voltammetry, electrochemical impedance spectroscopy, and contact angle measurements. The nature of the gold oxide layer was analyzed as a function of a potential holding in the aqueous double layer charge region at the interface between gold and the aqueous layer confined by insoluble organic solvents (hexane, chloroform, anisole, butyl acetate, xylene, and isopropyl ether). Different growth conditions change the homogeneity of the oxide layer leading to different passive properties. A synergetic effect on the gold oxidation of hydrogen dissolved in both the bulk metal and the confined aqueous layer is discussed.

The Electrochemical Surface Forces Apparatus: The Effect of Surface Roughness, Electrostatic Surface Potentials, and Anodic Oxide Growth on Interaction Forces, and Friction between Dissimilar Surfaces in Aqueous Solutions

We present a newly designed electrochemical surface forces apparatus (EC-SFA) that allows control and measurement of surface potentials and interfacial electrochemical reactions with simultaneous measurement of normal interaction forces (with nN resolution), friction forces (with μN resolution), and distances (with Å resolution) between apposing surfaces. We describe three applications of the developed EC-SFA and discuss the wide-range of potential other applications. In particular, we describe measurements of (1) force-distance profiles between smooth and rough gold surfaces and apposing self-assembled monolayer-covered smooth mica surfaces; (2) the effective changing thickness of anodically growing oxide layers with Å-accuracy on rough and smooth surfaces; and (3) friction forces evolving at a metal-ceramic contact, all as a function of the applied electrochemical potential. Interaction forces between atomically smooth surfaces are well-described using DLVO theory and the Hogg-Healy-Fuerstenau approximation for electric double layer interactions between dissimilar surfaces, which unintuitively predicts the possibility of attractive double layer forces between dissimilar surfaces whose surface potentials have similar sign, and repulsive forces between surfaces whose surface potentials have opposite sign. Surface roughness of the gold electrodes leads to an additional exponentially repulsive force in the force-distance profiles that is qualitatively well described by an extended DLVO model that includes repulsive hydration and steric forces. Comparing the measured thickness of the anodic gold oxide layer and the charge consumed for generating this layer allowed the identification of its chemical structure as a hydrated Au(OH)(3) phase formed at the gold surface at high positive potentials. The EC-SFA allows, for the first time, one to look at complex long-term transient effects of dynamic processes (e.g., relaxation times), which are also reflected in friction forces while tuning electrochemical surface potentials.

Impact of oxygen and argon plasma exposure on the roughness of gold film surfaces

The impact of oxygen and argon plasma exposure on the roughness of gold film Quartz Crystal Microbalance (QCM) electrodes is reported here, employing low levels of gas uptake and scanning tunneling microscope measurements to probe the post-exposure surface morphology. For equal exposure times, argon plasma bombardment is observed to produce both greater material removal and greater change in surface roughness. A possible explanation for this is that the oxygen plasma produces a protective gold oxide layer, which may remove the contaminants from the surface without creating defects in the gold surface. The result is also consistent with prior reports of chemical cleaning of the surface by reactive oxygen ions. Pentane gas adsorption on the argon bombarded QCM surfaces was, moreover, observed to occur at pressures that are several orders of magnitude lower than that for an unbombarded surface.

An Electrochemical DNA Biosensor Based on Gold Nanofilm and Stable Y Junction Structure

On the basis of gold nanofilm (GNF) electrode and Y junction structure of DNA, a simple and facile single-step DNA sensing protocol with improved sensitivity and lower detection limit was successfully developed. The GNF was pre- pared via rapid electrooxidization of the gold surface followed by the chemical reduction of the produced gold oxide layer. The capture probe DNA (c-DNA) was firstly immobilized onto GNF electrodes via Au—S bonding, for the subsequent forming of Y junction structure with target DNA (t-DNA) and reporter probe DNA (r-DNA) labeled with methylene blue (MB). Harnessing the unique properties of GNF would allow the improved contact of MB with the electrode surface and hence boost the interfacial electron communication. Experimental results of differential pulse voltammetry (DPV) showed that the peak current of the prepared biosensor was linear with the target DNA concentration from 1.0×10 -12 to 1.0×10 -9

Fabrication of nano-network gold films via anodization of gold electrode and their application in SERS

We report here a green and facile one-step method to fabricate nano-network gold films of low roughness via anodization of gold electrodes in an aqueous solution of l-ascorbic acid (AA) or hydroquinone (H2Q) at the oxidation peak potential. The preparation involves the formation of thin gold oxide layer by anodization of gold and its simultaneous and/or subsequent reduction by AA or H2Q. The as-fabricated nano-network gold films show very strong SERS activity in comparison with the substrates prepared by some other electrochemical roughening methods.

Characterization of Nanoporous Gold Electrodes for Bioelectrochemical Applications

The high surface areas of nanostructured electrodes can provide for significantly enhanced surface loadings of electroactive materials. The fabrication and characterization of nanoporous gold (np-Au) substrates as electrodes for bioelectrochemical applications is described. Robust np-Au electrodes were prepared by sputtering a gold-silver alloy onto a glass support and subsequent dealloying of the silver component. Alloy layers were prepared with either a uniform or nonuniform distribution of silver and, post dealloying, showed clear differences in morphology on characterization with scanning electron microscopy. Redox reactions under kinetic control, in particular measurement of the charge required to strip a gold oxide layer, provided the most accurate measurements of the total electrochemically addressable electrode surface area, A(real). Values of A(real) up to 28 times that of the geometric electrode surface area, A(geo), were obtained. For diffusion-controlled reactions, overlapping diffusion zones between adjacent nanopores established limiting semi-infinite linear diffusion fields where the maximum current density was dependent on A(geo). The importance of measuring the surface area available for the immobilization was determined using the redox protein, cyt c. The area accessible to modification by a biological macromolecule, A(macro), such as cyt c was reduced by up to 40% compared to A(real), demonstrating that the confines of some nanopores were inaccessible to large macromolecules due to steric hindrances. Preliminary studies on the preparation of np-Au electrodes modified with osmium redox polymer hydrogels and Myrothecium verrucaria bilirubin oxidase (MvBOD) as a biocathode were performed; current densities of 500 μA cm(-2) were obtained in unstirred solutions.

Formation and Growth of Oxide Layers at Platinum and Gold Nano- and Microelectrodes

The construction and characterization of platinum and gold disk electrodes with minimum radii of 7 nm (platinum) and 500 nm (gold) is reported. The electrodes were prepared with a micropipet puller using a two step procedure and have been characterized using scanning electron microscopy, scanning electrochemical microscopy, high speed chronoamperometry, and cyclic voltammetry. The formation and growth of platinum and gold oxide layers, on the electrodes at time scales from microseconds to seconds, is reported. Significantly, the apparent microscopic area as determined by forming and subsequently reducing an oxide layer in acidic electrolyte using cyclic voltammetry depends dramatically on the scan rate. While conventional roughness factors between 1.8 and 3 are observed on average for scan rates above 5 V s(-1), the apparent roughness can exceed 30 for scan rates less than 0.5 V s(-1). Chronoamperometry, conducted on the microsecond to millisecond time scale, is used to probe the dynamics of monolayer and multilayer oxide formation as well as the reversibility of the oxide formation and removal. The latter study suggests that (at least for platinum) the growth of the oxide layer proceeds with a lower constant rate after an oxide monolayer is formed.

Passivation of pinhole defect microelectrode arrays in ultrathin silica films immobilized on gold substrates

Nanoscopic defects present in ultrathin (~ 6 nm) silica films covalently attached to gold substrates through a gold oxide layer exhibit a voltammetric response consistent with a random array of ultramicroelectrodes. These pinholes can be passivated via electrochemical polymerization of phenol to create insulating poly(phenylene) oxide plugs as documented by atomic force microscopy and infrared reflectance-absorbance spectroscopy. Passivation of pinholes is ~ 99.5% complete after 550 voltammetric cycles of oxidative electropolymerization.

An optimised electrode pre-treatment for SAM formation on polycrystalline gold

The ability to fabricate good-quality self-assembled layers on polycrystalline gold depends sensitively on the reliable removal of contaminants inherent on ambient surfaces and the subsequent exposure of a pristine gold surface. We present here a simple method of reducing the gold oxide layer formed by the extensive oxidative treatments commonly utilised for surface cleaning. The method consists of an electrochemical reduction in the same electrolyte as used for electrochemical polishing and is demonstrated herein for both saturated alkyl and ethylene glycol thiol SAMs. The protocol is highly reproducible, and a more effective and rapid means of ensuring high chemisorbed molecular densities than chemical pre-treatment.

DFT study of the Au(3 2 1) surface reconstruction by consecutive deposition of oxygen atoms

The consecutive deposition of oxygen atoms on the Au(3 2 1) surface was studied carrying out calculations based in the spin polarized density functional theory within the GGA/PW91 exchange-correlation functional. The clean and oxygen covered Au(3 2 1) surfaces were modeled by a periodic supercell approach; the unit cell has 15 gold atoms, the outermost five define the (1 1 1) terrace and the step. The adsorption of a single oxygen atom is more favorable if it occurs at a fcc hollow site on the (1 1 1) terrace adjacent to the step while for the co-adsorption of two oxygen atoms on the Au(3 2 1) surface, hollow sites nearby the step are preferred. The introduction of an additional oxygen atom on the already optimized slabs containing two oxygen atoms yield, in some cases, structures with a single oxygen atom attached to the surface and an oxygen molecule far from the slab. In the other cases, several initial geometries converged to the same final structure and, in general, the adsorption of three oxygen atoms per unit cell was found to be thermodynamically unfavorable. The exception was a planar structure formed after reconstruction of the surface. The simultaneous adsorption of four oxygen atoms per unit cell is characterized by relatively high adsorption energies of −0.53 and −0.56 eV, corresponding to a porous structure containing sub-surface oxygen atoms forming a gold oxide layer and to a well ordered folded structure. The addition of a fifth oxygen atom to the structures already containing four oxygen atoms on the surface resulted in an endothermic processes suggesting that they will be hard to found even after exposure to high pressures of oxygen.

Ultrathin Silica Films Immobilized on Gold Supports: Fabrication, Characterization, and Modification

A novel method for covalent attachment of ultrathin silica films (thickness <10 nm) to gold substrates is reported. Silica layers were prepared using spin-coating of sol-gel precursor solutions onto gold substrates that were cleaned and oxidized using UV photo-oxidation in an ozone atmosphere. The gold oxide layer resulting from this process acts as a wetting control and adhesive agent for the ultrathin silica layer. Control of silica layer thickness between approximately 6 and 60 nm through modification of precursor solution composition or by repetitive deposition is demonstrated. Films were characterized using infrared spectroscopy, ellipsometry, atomic force microscopy, and cyclic voltammetry. For the standard deposition parameters developed here, films were determined to be 5.5 +/- 0.75 nm thick, and were stable in aqueous solutions ranging in pH from 2 to 10 for at least 30 min. Films contained nanoscopic defects with radii of <or=2 microm in all cases, at a density of 1.6 defects per 10 mum x 10 mum region, with a weighted average defect radius of 200 nm. This array of defects was shown to exhibit a voltammetric response consistent with an ultramicroelectrode array. Finally, ultrathin silica films were also modified with self-assembled monolayers of octadecylsilane, demonstrating the surface chemistry of bulk silica.

Triggering blue–red transition response in polydiacetylene vesicles: an electrochemical surface plasmon resonance method

Electrochemical surface plasmon resonance (E-SPR) was used to investigate whether the chromic properties of a polydiacetylene (PDA) vesicle films, adsorbed onto an ultra-thin gold electrode, could be triggered by applied potential. This approach constitutes a preliminary model for a novel approach to the use of a triggered chromic transition, as an indicator of biorecognition headgroup binding in these materials. A PDA chromic blue-red transition was identified in E-SPR against the background Deltaepsilon(e) and Deltaepsilon(m). The latter resulted in a ca. 100 mDeg V(-1) shift in the SPR minimum, in the presence of PDA, with the PDA shielding changes in epsilon(e). Electrochemical charge transfer processes in the pre-oxide/oxide anodic region with adsorbed oxygen and hydroxide, involving a change in Au redox state (Au(0)/Au(+)) were visible in the SPR, due to a change in the gold layer thickness and gold oxide layer. However, the cathodic processes, not involving a change in the Au redox state or a increase/decrease in the surface layer dielectric, did not cause a change in the SPR. Based on this, dramatic changes in the optical properties of the adsorbed PDA film could be triggered at an applied cathodic potential, and were identified using SPR. These correlated with a pH-induced chromic transition. Both protonation and ion binding, linked with headgroup environment, were implicated in causing structural transitions in the adsorbed vesicle layer that may also be linked with their bulk optical properties.