Nanoporous Gold Layer Research Articles

High-Temperature Melt Stamping of Polymers Using Polymer/Nanoporous Gold Composite Stamps.

Parallel lithographic deposition of polymers onto counterpart substrates is a widely applied surface manufacturing operation. However, polymers may only be soluble in organic solvents or are insoluble at all. Solvent evaporation during stamping may trigger hardly controllable capillarity-driven flow processes or phase separation, and polymer solutions may spread on the counterpart substrates. Solvent-free stamping of melts prevents these drawbacks. Here, a stamp design for the deposition of melts is devised, which intrinsically circumvents ink depletion. The stamps' topographically patterned contact surfaces with protruding contact elements contacting the counterpart substrates consist of a nanoporous gold layer with a thickness of a few micrometers. The nanoporous gold layer is attached to a molten polymer layer, which is support for the nanoporous gold layer and ink reservoir at the same time. The nanoporous gold layer in turn stabilizes the topography of the stamps' contact surfaces. As examples, arrays of submicron microdots of polystyrene and poly(vinylidenefluoride-trifluoroethylene) (PVDF-TrFE) are manufactured. The P(VDF-TrFE) microdots are partially crystalline, ferroelectric, and can be locally poled. It is envisioned that the methodology reported here can be automatized and may be extended to functional low-molecular-mass compounds, such as active pharmaceutical ingredients.

A 2D-to-3D morphology transitions of gold in organic acid electrolytes: Characterization and application in bioanode design

The inexpensive, relatively fast and scalable production of nanostructured electrodes may benefit future disposable biosensor technology. In this research, nanoporous gold (NPG) surfaces were successfully fabricated by anodizing the gold in different organic acid, and further used for glucose dehydrogenase (GDH)-based bioanode design. The porosity of NPG layers was manipulated by changing the anodization time, and the dependency of relative electrochemical surface area (rESA) on the bioanode performances was systematically examined. The X-ray photoelectron spectroscopy (XPS) studies of NPG formed in oxalic acid (NPGox) and their depth profiles of Au 4f and O 1s evidenced the chemical state of pure metallic gold (Au0), whereas NPG formed in glycolic acid (NPGgly) has appreciable content of gold oxide and hydroxides. The NPGgly/4-ATP/GDH bioanodes exhibited direct electron transfer (DET-type) bioelectrocatalytic activity, while the highest current densities of 0.35 mA cm−2 and low onset potential of −0.189 V (vs. Ag/AgCl) were observed. The electrodes showed long-term stability and, up to three weeks, retained 76 % of their initial value. Furthermore, the biofuel cell possessed the maximum power densities of 31.8 µW cm−2 and open-circuit potential (OCP) differences of 0.441 V, thus evidencing that NPGgly layers could be encouraging for developing biosensors and biofuel cells.

Metal-Organic Framework-Decorated Nanochannel Electrode: Integration of Internal Nanoconfined Space and Outer Surface for Small-Molecule Sensing.

Ionic current measurement has been the dominant signaling strategy in nanochannel-based sensors. However, the direct probing of the capture of small molecules is still challenging, and the sensing potential of the outer surface of nanochannels is always ignored. Here, we report the fabrication of an integrated nanochannel electrode (INCE) with nanoporous gold layers modified on two sides of nanochannels, and its application for small-molecule analysis was explored. Metal-organic frameworks (MOFs) were decorated inside and outside of nanochannels, enabling the reduction of pore size to several nanometers, which is among the thickness range of the electric double layer for confined ion diffusion. Combined with excellent adsorption characteristics of MOFs, the developed nanochannel sensor successfully constructed the internal nanoconfined space that could directly capture small molecules and instantly generate a current signal. The contribution of the outer surface and the internal nanoconfined space to diffusion suppression to electrochemical probes was investigated. We found that the constructed nanoelectrochemical cell was sensitive in both the inner channel and the outer surface, signifying a novel sensing mode with integration of the internal nanoconfined space and the outer surface of nanochannels. The MOF/INCE sensor showed excellent performance toward tetracycline (TC) with a detection limit of 0.1 ng·mL-1. Subsequently, sensitive and quantitative detection of TC down to 0.5 μg·kg-1 was achieved in actual chicken samples. This work may open up a new model of nanoelectrochemistry and provide an alternative solution in the field of nanopore analysis for small molecules.

Enzymatic Catalysis in Size and Volume Dual-Confined Space of Integrated Nanochannel-Electrodes Chip for Enhanced Impedance Detection of Salmonella.

Nanochannel-based confinement effect is a fascinating signal transduction strategy for high-performance sensing, but only size confinement is focused on while other confinement effects are unexplored. Here, a highly integrated nanochannel-electrodes chip (INEC) is created and a size/volume-dual-confinement enzyme catalysis model for rapid and sensitive bacteria detection is developed. The INEC, by directly sandwiching a nanochannel chip (60µm in thickness) in nanoporous gold layers, creates a micro-droplet-based confinement electrochemical cell (CEC). The size confinement of nanochannel promotes the urease catalysis efficiency to generate more ions, while the volume confinement of CEC significantly enriches ions by restricting diffusion. As a result, the INEC-based dual-confinement effects benefit a synergetic enhancement of the catalytic signal. A 11-times ion-strength-based impedance response is obtained within just 1min when compared to the relevant open system. Combining this novel nanoconfinement effects with nanofiltration of INEC, a separation/signal amplification-integrated sensing strategy is further developed for Salmonella typhimurium detection. The biosensor realizes facile, rapid (<20min), and specific signal readout with a detection limit of 9 CFU mL-1 in culturing solution, superior to most reports. This work may create a new paradigm for studying nanoconfined processes and contribute a new signal transduction technique for trace analysis application.

From Chip Size to Wafer-Scale Nanoporous Gold Reliable Fabrication Using Low Currents Electrochemical Etching.

We report a simple, scalable route to wafer-size processing for fabrication of tunable nanoporous gold (NPG) by the anodization process at low constant current in a solution of hydrofluoric acid and dimethylformamide. Microstructural, optical, and electrochemical investigations were employed for a systematic analysis of the sample porosity evolution while increasing the anodization duration, namely the small angle X-ray scattering (SAXS) technique and electrochemical impedance spectroscopy (EIS). Whereas the SAXS analysis practically completes the scanning electronic microscopy (SEM) investigations and provides data about the impact of the etching time on the nanoporous gold layers in terms of fractal dimension and average pore surface area, the EIS analysis was used to estimate the electroactive area, the associated roughness factor, as well as the heterogeneous electron transfer rate constant. The bridge between the analyses is made by the scanning electrochemical microscopy (SECM) survey, which practically correlates the surface morphology with the electrochemical activity. The results were correlated to endorse the control over the gold film nanostructuration process deposited directly on the substrate that can be further subjected to different technological processes, retaining its properties. The results show that the anodization duration influences the surface area, which subsequently modifies the properties of NPG, thus enabling tuning the samples for specific applications, either optical or chemical.

A novel electrochemical sensor for determination of hydroxyl radicals in living cells by coupling nanoporous gold layer with self-assembled 6-(Ferrocenyl) hexanethiol

The detection of hydroxyl radicals (•OH) in live cells is significant to study its physiological and pathological roles, while it is full of challenge due to the extremely low concentration and short lifetime of •OH. Herein, we have developed a novel electrochemical sensor based on 6-(Ferrocenyl) hexanethiol (6-FcHT) self-assembled nanoporous gold layer (NPGL) modified GE (6-FcHT/NPGL/GE), which can detect the release of •OH from living cells with high sensitivity and selectivity. The superior sensitivity can stem from the unique porous architecture of NPGL, which enlarged electrode surface area and expedited electron transportation during electrochemical reactions. Additionally, NPGL provides more active binding sites for the assembly of capture agent (6-FcHT) of •OH, thus ensuring high selectivity. For comparison, 6-FcHT/GE was applied to detect •OH, and the obtained sensitivity was 0.0305 mA nM−1 and detection limit was 0.133 nM in the linear range of 0.4 nM–70 nM. After modification of NPGL, the sensitivity of 6-FcHT/NPGL/GE to the •OH response was increased to 0.1364 mA nM−1, detection limit was reduced to 0.316 pM and the linear range was extended from 1 pM to 100 nM. It is worth mentioning that a plenty of extra merits has also been validated like reproducibility, repeatability and stability, enabling to direct electrochemical detection of •OH in HepG2 cells.

Decoupling the role of stress and corrosion in the intergranular cracking of noble-metal alloys.

Intergranular stress-corrosion cracking (IGSCC) is a form of environmentally induced crack propagation causing premature failure of elemental metals and alloys. It is believed to require the simultaneous presence of tensile stress and corrosion; however, the exact nature of this synergy has eluded experimental identification. For noble metal alloys such as Ag-Au, IGSCC is a consequence of dealloying corrosion, forming a nanoporous gold layer that is believed to have the ability to transmit cracks into grain boundaries in un-dealloyed parent phase via a pure mechanical process. Here using atomic-scale techniques and statistical characterizations for this alloy system, we show that the separate roles of stress and anodic dissolution can be decoupled and that the apparent synergy exists owing to rapid time-dependent morphology changes at the dealloyed layer/parent phase interface. We discuss the applicability of our findings to the IGSCC of important engineering Fe- and Ni-based alloys in critical applications.

Electrochemically synthesized nanoporous gold as a cathode material for Li-O2 batteries

Nanoporous gold (NPG) prepared via chemical de-alloying has been recently shown to dramatically improve the reversibility and kinetics of Li-O2 batteries, but high cost makes its use as practical electrode material difficult. Recently developed electrochemical routines for synthesis of very thin NPG layers (<100 nm) on various low-cost substrates could potentially provide a feasible economic alternative. In this work, NPG on both gold and glassy carbon (GC) substrates was successfully synthesized via electrochemical de-alloying method and tested as cathode material in Li-O2 batteries. The results show that electrochemically synthesized NPG cathode cycles repeatedly with LiFePO4 anode. The voltage hysteresis is also significantly reduced when NPG is used in comparison with plain GC. Along with these results, challenges that need to be addressed for future implementation of NPG cathode in practical Li-O2 batteries are also discussed.

Effect of Temperature and Chloride Concentration on the Anodic Formation of Nanoporous Gold Films in Chloride Solutions

The preparation of nanoporous gold (NPG) has been the subject of extensive research, and the anodization of Au in chloride solutions has received recent attention as an efficient and time‐saving method. In the present work, the effects of reaction temperature and Cl– concentration on the anodic formation of NPG structures were investigated. The anodization efficiency was evaluated in terms of the roughness factor (R f) of NPG layers, which varied as a function of these two reaction parameters. The R f of NPG gradually increased with temperature up to 40 °C and then leveled off at higher temperatures. In terms of Cl– concentration, 1.0 M yielded the maximum R f of the NPG. The dependence of the anodization efficiency on the two reaction parameters was examined with regard to the electrochemical dissolution of Au as well as the formation of Au–Cl complexes and protective surface oxide layers. The mechanistic aspects of the anodic formation of NPG give insight into the efficient preparation of highly porous NPG structures.

Improved biomolecular detection based on a plasmonic nanoporous gold film fabricated by oblique angle deposition.

We demonstrated an enhanced surface plasmon resonance (SPR) detection by incorporating a nanoporous gold film on a thin gold substrate. Nanoscale control of thickness and roughness of the nanoporous layer was successfully accomplished by oblique angle deposition. In biosensing experiments, the results obtained by biotin-streptavidin interaction showed that SPR samples with a nanoporous gold layer provided a notable sensitivity improvement compared to a conventional bare gold film, which is attributed to an excitation of local plasmon field and an increased surface reaction area. Imaging sensitivity enhancement factor was employed to estimate an overall sensor performance of the fabricated samples and an optimal SPR structure was determined. Our approach is intended to show the feasibility and extend the applicability of the nanoporous gold film-mediated SPR biosensor to diverse biomolecular binding events.

Improved Anodic Stripping Voltammetric Determination of Arsenic Using Nanoporous Gold Microelectrode

Arsenic is a highly toxic element and a relatively widespread water pollutant which is thought to be poisoning significant amount of people in various developing countries [1]. Exposure to arsenic can cause a variety of adverse health effects, including dermal changes and respiratory, cardiovascular, gastrointestinal, genotoxic, mutagenic, and carcinogenic effects. Contamination of groundwater by arsenic has been reported in 20 countries where arsenic levels in drinking water are above the World Health Organization’s arsenic guide line value of 10 mg L-1. Many detection methods have been developed for determination of such levels of arsenic. These include inductively coupled plasma mass spectrometry (ICPMS), graphite furnace atomic absorption spectrometry, and high-performance liquid chromatography with ICPMS. However, the most reliable techniques are more suitable for laboratory conditions only and are time-consuming. They cannot thus be used for routine in-field monitoring of a large number of samples. Electrochemical techniques provide a possible means to this end since they are both rapid and portable. We have developed a fast and convenient electrochemical alloying-dealloying approach to fabricate a range of nanoporous microelectrodes of noble metals by applying modulated potential to their corresponding disk microelectrodes in an ionic liquid electrolyte bath comprising of ZnCl2 and 1-ethyl-3-methylimidazolium chloride [3]. The resulted nanoporous microelectrodes have high surface area and retain diffusional properties typical of a regular microelectrode. In this work, we fabricated nanoporous gold (NPG) microelectrodes by the electrochemical alloying-dealloying method and investigated their electrochemical responses to arsenic (III) in acidic media using anodic stripping voltammetry. Figure 1(A) shows the image of surface layers of the NPG microelectrode which was measured using field-emission environmental scanning electron microscope (SEM). Regular nanopores (black regions) and ligament spacings (white regions) were observed. To examine the ability of the NPG microelectrode as a tool in arsenic detection in water, linear sweep voltammetry and 0.1 M HNO3 as supporting electrolyte were used. Deposition of As(III) onto the NPG and an Au-disk microelectrodes was carried out at deposition time of 120 s from the electrolyte solution containing As(III) during anodic stripping voltammetric measurements. Figure 1(B) shows that the stripping peak measured is considerably higher than on the NPG microelectrode than the Au-disk microelectrode of the same geometric diameter. We further measured the limit of detection (LOD) for both the NPG and Au-disk microelectrodes. Their LOD values are around 2 mg L-1, lower than the he WHO recommended level 10 mg L-1. Insert Figure 1 Figure 1. (A) SEM image of nanoporous gold layer. (B) Anodic stripping voltammograms measured at 100 mV s-1 after deposition of 120 s at -0.40 V vs Ag/AgCl from 0.1 M HNO3containing As(III) for the NPG (blue) and Au-disk (red) microelectrodes.

Characterisation of Nanoporous Gold for Use in a Dissolved Oxygen Sensing Application

A nanoporous gold (NPG) microdisc array in a three-electrode electrochemical cell arrangement has been fabricated and characterised for a dissolved oxygen sensing application. The device will be used for monitoring of drinking water quality in rural communities in India, where there is a need for a miniaturised, low-cost dissolved oxygen sensor. The NPG microdisc array exhibits improved electrochemical responses compared to a planar gold microdisc array, owing to its extended surface area. In 1 mM ferrocene carboxylic acid, the experimental limiting currents obtained are twice as large as the theoretical limiting current for a planar gold microarray. A 252-nm-thick NPG layer resulted in a limiting current of 627 nA (compared to the estimated current of 295 nA). The voltammetric behaviour in water samples from a pilot site in India was investigated for increasing oxygen content, and a comparison was made with a macrodisc gold electrode and an unmodified microdisc array. The NPG microdisc array demonstrated improved characteristics over both devices and exhibited a higher correlation (average value 0.925) compared with the unmodified array (0.819) and macrodisc (0.870). An improvement in signal-to-noise ratio was also observed to be 12.349 versus 5.259 (microelecrode array) and 5.604 (macroelectrode). The response time for the NPG microdisc array versus a commercial dissolved oxygen sensor showed a faster response time of 20 versus 40 s.

Enhanced Adhesion of Ultrathin Nanoporous Au Deposits by Electrochemical Oxidation of Glassy Carbon

Electrochemical (EO) and Thermochemical (TO) oxidation have been considered as means for improved adhesion of metals to glassy carbon (GC) substrates. This paper is aimed at studying the effect of EO on the all-electrochemical processing of nanoporous gold (NPG) catalysts with emphasis on their adhesion to a GC substrate. It has been shown by two independent assessment approaches that EO improves twice the amount of retained NPG on GC helping to keep about 90% originally existing material after a two-hour rotation disk electrode test at 1600 rpm. This work also demonstrates that EO modifies the GC in a way making it more passive in the alloy deposition and subsequent dealloying processes. This effect results in a progressive nucleation density decrease with the depth of oxidation which in turn leads to a lack of continuity of the NPG layer. This trend is also accompanied by a decrease in deposition efficiency as a substantial part of the cathodic current is spent on the reduction of the pre-oxidized GC surface. Our study identifies the best outcome of the trade-off scenario between nucleation density, deposition efficiency and adhesion strength to be associated with depth of oxidation in the range 0.10 to 0.25 μm.

Galvanic synthesis of three-dimensional and hollow metallic nanostructures.

We report a low-cost, facile, and template-free electrochemical method of synthesizing three-dimensional (3D) hollow metallic nanostructures. The 3D nanoporous gold (3D-NPG) nanostructures were synthesized by a galvanic replacement reaction (GRR) using the different reduction potentials of silver and gold; hemispherical silver nanoislands were electrochemically deposited on cathodic substrates by a reverse-pulse potentiodynamic method without templates and then nanoporous gold layer replicated the shape of silver islands during the GRR process in an ultra-dilute electrolyte of gold(III) chloride trihydrate. Finally, the wet etching process of remaining silver resulted in the formation of 3D-NPG. During the GRR process, the application of bias voltage to the cathode decreased the porosity of 3D-NPG in the voltage range of 0.2 to -0.62 V. And the GRR process of silver nanoislands was also applicable to fabrication of the 3D hollow nanostructures of platinum and palladium. The 3D-NPG nanostructures were found to effectively enhance the SERS sensitivity of rhodamine 6G (R6G) molecules with a concentration up to 10-8 M.Electronic supplementary materialThe online version of this article (doi:10.1186/1556-276X-9-679) contains supplementary material, which is available to authorized users.

Enhanced Adhesion of Continuous Nanoporous Au Layers by Thermochemical Oxidation of Glassy Carbon

The fabrication of a nanoporous gold (NPG)-based catalyst on a glassy carbon (GC) support results normally in large isolated and poorly adhering clusters that suffer considerable material loss upon durability testing. This work exploits thermochemical oxidation of GC, which, coupled with the utilization of some recent progress in fabricating continuous NPG layers using a Pd seed layer, aims to enhance the adhesion to the GC surface. Thermochemical oxidation causes interconnected pores within the GC structure to open and substantially improves the wettability of the GC surface, which are both beneficial toward the improvement of the overall quality of the NPG deposit. It is demonstrated that thermochemical oxidation neither affects the efficiency of the Au0.3Ag0.7 alloy (NPG precursor) deposition nor hinders the achievement of continuity in the course of the NPG fabrication process. Furthermore, adhesion tests performed by a rotating disk electrode setup on deposits supported on thermochemically-oxidized and untreated GCs ascertain the enhanced adhesion on the thermochemically-oxidized samples. The best adhesion results are obtained on a continuous NPG layer fabricated on thermochemically-oxidized GC electrodes seeded with a dense network of Pd clusters.

Effect of pH on Anodic Formation of Nanoporous Gold Films in Chloride Solutions: Optimization of Anodization for Ultrahigh Porous Structures

Nanoporous gold (NPG) structures have useful applications based on their unique physical and chemical properties; therefore, the development of NPG preparation methods is the subject of extensive research. Recently, the anodization of Au surfaces was suggested as an efficient method for preparing porous Au structures. In this work, the mechanistic aspects of the anodization of Au in Cl(-)-containing solutions for the preparation of NPG layers were investigated. The effects of the experimental parameters of the anodization reaction on the porosity of the NPG layers in terms of the roughness factor (Rf) were examined. The anodic formation of NPG was more effective in buffered solutions than in unbuffered electrolytes. The Rf of the NPG layer was sensitive to the electrolyte pH; this was ascribed to the efficient formation of protecting layers of gold oxide on the newly formed NPG structures. In buffer solutions at pH 8, ultrahigh porous NPG layers with Rf values of 1300 were obtained within 15 min. The ultrahigh porous NPG layers were used for the electrochemical detection of glucose; a high sensitivity of 135 μA mM(-1) cm(-2) was achieved in the presence of 0.1 M Cl(-). This straightforward and time-saving preparation of NPG surfaces will provide new opportunities for applications of NPG structures.

Nanoporous gold microelectrode prepared from potential modulated electrochemical alloying–dealloying in ionic liquid

Nanoporous gold (NPG) microelectrodes with high surface area and open pore network were successfully prepared by applying modulated potential to a polycrystalline Au-disk microelectrode in ionic liquid electrolyte containing ZnCl2 at elevated temperature. During cathodic process, Zn is electrodeposited and interacted with Au microdisk substrate to form a AuZn alloy phase. During subsequent anodic process, Zn is selectively dissolved from the alloy phase, leading to the formation of a NPG layer which can grow with repetitive potential modulation. Scanning-electron microscope and energy dispersive X-ray microscope measurements show that the NPG microelectrodes possessing nanoporous structures can be tuned via potential modulation, and chemically contain a small amount of Zn whose presence has no obvious influence on electrochemical responses of the electrodes. Steady-state and cyclic voltammetric studies suggest that the NPG microelectrodes have high surface area and keep diffusional properties of a microelectrode. Electrochemical nitrite reduction and oxidation are studied as model reactions to demonstrate potential applications of the NPG microelectrodes in electrocatalysis and electroanalysis. These facts suggest that the potential-modulated electrochemical alloying/dealloying in ionic liquid electrolyte offers a convenient green-chemistry method for the preparation of nanoporous microelectrodes.

Nanoporous gold—Application to extraordinary optical transmission of light

The authors present their work in the preparation of nanoporous gold layers and their patterning with an original procedure preserving the porosity, to obtain the phenomenon of extraordinary transmission of light with a porous material. The design, fabrication, and characterization of nanoslit arrays made with bulk gold and nanoporous gold films are presented and their sensing performances are compared after coating with thiolated organic molecules. Thanks to a greatly enhanced surface-to-volume ratio, nanoporous gold reveals benefits for better reaction efficiency and detection sensitivity. Moreover, plasmonic properties in the near-IR range assure employment in plasmonic devices.

RETRACTED: Fabrication of stratified nanoporous gold for enhanced biosensing

By a dealloying/annealing/redealloying strategy, nanoporous gold (NPG) with hierarchical microstructure is fabricated for electrochemical biosensing application. The first dealloying and annealing would produce NPG/AuAg alloy composite with a large-pore NPG layer and the second dealloying would further etch the AuAg alloy part in the composite, generating a small-pore NPG layer. By using the large-pore (≈ 100 nm) layer as the glucose oxidase (GOx) container, and the small-pore (≈ 12 nm) layer as a signal producer, this novel hierarchical NPG is demonstrated to be a good support for enzyme immobilization and fabricating enzyme-based biosensors. The immobilized GOx retains ≈ 92% of the initial activity after 7 repeated use. The GOx-loaded stratified NPG biosensor can detect glucose more sensitively with a wider linear range (up to 22 mM) than normal NPG with a uniform pore size of 30-40 nm (linear range: up to 17 mM).

Nanoporous gold plasmonic structures for sensing applications

The fabrication, characterization and functionalization of periodically patterned nanoporous gold layers is presented. The material shows plasmonic properties in the near infrared range, with excitation and propagation of surface plasmon polaritons. Functionalization shows a marked enhancement in the optical response in comparison with evaporated gold gratings, due to a great increase of the active surface. Due to its superior response, nanoporous gold patterns appear promising for the realization of compact plasmonic platforms for sensing purposes.