Nanoporous Gold Electrode Research Articles

Nanoporous gold based electrodes for electrochemical studies of human neuroglobin

Human neuroglobin (Ngb) is a heme protein expressed in the brain tissue and retina with an important neuroprotective role. However, its molecular mechanism is still under exploration. To gain further insight into this issue, we developed an electrochemical strategy using a facilely prepared nanoporous gold (NPG) electrode. Immobilization of Ngb onto the NPG surface enabled rapid direct electron transfer between Ngb and the electrode, whereas less electrochemical response was generated using a planar gold electrode. Voltammograms recorded using the NPG electrodes gave an E0′ value of −0.170 V (vs. standard hydrogen electrode (SHE)), which is in good agreement with the previously reported values. Furthermore, a heterogeneous electron transfer rate constant of 46 s−1 was obtained, which was larger than the rate constants reported previously. The addition of ferric cytochrome c (Cyt c), a potential redox partner protein of Ngb, increased the cathodic current in the voltammogram, indicating the electron transfer from the reduced (ferrous) Ngb to the ferric Cyt c. In addition, mutational studies demonstrated for the first time that the disulfide bond formation between Cys46 and Cys55 is important for this electron transfer. These results may indicate that ferrous Ngb, in addition to the ferric form, also contributes to the anti-apoptotic properties of Ngb, depending on the disulfide bond formation. Thus, the NPG-based electrochemical system developed in our study is an effective tool for uncovering the molecular basis of Ngb coupled with electron transfer.

Electrochemical preparation of Pt nanoparticles modified nanoporous gold electrode with highly rough surface for efficient determination of hydrazine

A highly sensitive electrochemical sensing platform for hydrazine was developed based on a highly surface-roughened nanoporous gold electrode decorated with Pt nanoparticles (Pt/HNPG). The self-supporting composite electrode is binder-free, which was fabricated via anodization of a commercial AuSn alloy in HCl electrolyte and subsequent electrodeposition of H2PtCl6 precursor in aqueous solution. The morphology and composition of Pt/HNPG electrode were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The catalytic activity of Pt/HNPG towards electrooxidation of hydrazine was examined. It is found that both Pt nanoparticles loadings and solution pH have strong influence on the electrocatalytic activity. At the potential of –0.1 V (vs. SCE), the fabricated composite electrode showed excellent sensing performance for hydrazine with a wide linear range of 5 μM to 6.105 mM, a low detection limit of 1.03 μM (signal-to-noise ratio (S/N) of 3) and a high sensitivity of 3449.68 μA mM−1 cm−2. Moreover, the sensor exhibited good selectivity, repeatability and stability. It was also successfully applied for the highly sensitive determination of hydrazine in real water samples with satisfactory results.

Optimized deposition time boosts the performance of Prussian blue modified nanoporous gold electrodes for hydrogen peroxide monitoring

Prussian blue (PB) modified nanoporous gold (NPG) electrodes exhibit great potential for improving the detection sensitivity and stability for hydrogen peroxide monitoring. The NPG provides large surface-to-volume ratio as well as diffusion ‘highways’ to assist the transfer of the ions. In the present work, we optimized the deposition time for NPG fabrication and examine the electrochemical performance of the electrodes. A critical deposition time on the electrochemical performances including linear range, operational stability and sensitivity was experimentally determined. Below and above such a deposition time, two different growth patterns of the microstructures were observed. This transition of deposited structures corresponding to the critical time results in different pathways for electron transfer and ion diffusivity through PB lattice.

Flexible Nanoporous Gold Electrodes for Electroanalysis in Complex Matrices

AbstractFlexible, three‐dimensional nanoporous gold (NPG) electrodes were fabricated on polydimethylsiloxane (PDMS) surfaces without any functionalization, and their electrochemical properties examined in the absence and presence of common biofouling agents using cyclic voltammetry and redox potentiometry. In this work, potassium ferricyanide and fibrinogen were used as a model redox probe and denaturing agent, respectively, to demonstrate that flexible NPG electrodes behave ideally and can make electrochemical measurements in biofouling solutions. Using cyclic voltammetry, the non‐faradaic capacitive current was shown to be directly proportional to the scan rate, while the redox chemistry of potassium ferricyanide in the presence and absence of fibrinogen demonstrated reversible voltammetry. Using redox potentiometry, Nernst plots for the potassium ferri‐/ferrocyanide redox system gave rise to an expected Nernstian slope of 60 mV. These flexible electrodes could be particularly beneficial in electrochemical sensing in complex environments. Proof of concept was demonstrated through the measurement of uric acid in red blood cell packets using cyclic voltammetry.

Anomalous Trends in Nucleic Acid-Based Electrochemical Biosensors with Nanoporous Gold Electrodes.

Molecular diagnostics have significantly advanced the early detection of diseases, where electrochemical sensing of biomarkers has shown considerable promise. For a nucleic acid-based electrochemical sensor with signal-off behavior, the performance is evaluated by percent signal suppression (% ss), which indicates the change in current after hybridization. The % ss is generally due to more redox molecules (e.g., methylene blue) associating with the probe DNA bases in the single-strand form than the double-strand form upon hybridization with the target nucleic acid. Nanostructured electrodes generally enhance electrochemical sensor performance via several mechanisms, including increased number of capture probes per electrode volume and unique nanoscale transport phenomena. Here, we employ nanoporous gold (np-Au) as a model electrode material to study the influence of probe immobilization solution concentration on sensor performance and the underlying mechanisms. Unlike planar gold (pl-Au) electrodes, where % ss reaches a steady state with increasing concentration of the grafting solution, the % ss displays peak performance at certain grafting solution concentrations followed by rapid deterioration and reversal of the % ss polarity, suggesting an unexpected case of increased charge transfer upon hybridization. Fluorometric assessments of electrochemically desorbed nucleic acids for different electrode morphologies reveal that a significant amount of DNA molecules (unhybridized and hybridized) remain within the nanopores posthybridization. Analysis of electrochemical signals (e.g., square wave voltammogram shape) suggests that the large unbound nucleic acid concentration may be altering the modes of methylene blue interaction with the nucleic acids and charge transfer to the electrode surfaces.

Nanoporous gold electrode for ultrasensitive detection of neurotoxin fasciculin

Acetylcholinesterase (AChE), an efficient biocatalyst known to hydrolyze the neurotransmitter acetylcholine, could be inactivated in the presence of insecticides, nerve agents or other drug inhibitors to thus result in disrupted neurotransmission. Improvement in the peripheral cholinergic function, as well as overall cognition and neuronal functions of an exposed system could be achieved if the mechanisms of inhibitions are deactivated in a controlled fashion and with rapid response time. Herein, we proposed to develop a simple AChE biosensor capable to realize the rapid detection of neurotoxins. Our approach uses a nanoporous gold film (NPGF) and reduced graphene oxide-tin dioxide nanoparticle (RGO-SnO2) nanocomposite to define the highly active electrode interface where the electrochemical monitoring of the interaction between AChE and its target molecule, fasciculin, could take place. Our results demonstrate that the established biosensor had the ability to monitor fasciculin concentrations at the ultra-low limit of detection of 8 pM, an inhibition rate of 8% and within only 30min of electrochemical exposure. Our study provides a convenient technology for the rapid and ultrasensitive detection of neurotoxins and has the potential for large applicability to other drugs or toxins screening.

Simultaneous electrochemical sensing of ascorbic acid and uric acid under biofouling conditions using nanoporous gold electrodes

Direct electrochemical sensing of biomolecules in samples with complex matrices such as human serum and blood is challenged by sensor surface biofouling resulting from the adsorption of concomitant macromolecules, particularly proteins. In this research, nanoporous gold (NPG) electrodes prepared from dealloying of a low-cost silver‑gold alloy are applied as antibiofouling electrodes for the simultaneous determination of ascorbic acid (AA) and uric acid (UA). From cyclic voltammetric assays, the antibiofouling features of the fabricated NPG were interrogated in the presence of biofouling agents, fibrinogen (FN) and bovine serum albumin (BSA) proteins using ascorbic acid and uric acid as the redox probes. It was shown that the porous surface of NPG ensured a stable signal for the redox probe even in the presence of proteins found in biological samples. The antibiofouling capability of NPG is credited to the biosieving effect of the morphology of the NPG, restricting the diffusion of large proteins into gold pores while permitting access of small electroactive molecules to efficiently exchange electrons. By applying differential pulse voltammetry (DPV) under optimum conditions, the AA and UA biomolecules were simultaneously determined with limits of detection of 63.0 and 9.0μM, respectively. The DPV signals of AA and UA were stable on NPG in a biofouling environment and, the proposed NPG sensor was successfully applied for the simultaneous determination of AA and UA in the mimic human serum sample of fetal bovine serum (FBS).

3D Patterning of Si by Contact Etching With Nanoporous Metals.

Nanoporous gold and platinum electrodes are used to pattern n-type silicon by contact etching at the macroscopic scale. This type of electrode has the advantage of forming nanocontacts between silicon, the metal and the electrolyte as in classical metal assisted chemical etching while ensuring electrolyte transport to and from the interface through the electrode. Nanoporous gold electrodes with two types of nanostructures, fine and coarse (average ligament widths of ~30 and 100 nm, respectively) have been elaborated and tested. Patterns consisting in networks of square-based pyramids (10 × 10 μm2 base × 7 μm height) and U-shaped lines (2, 5, and 10 μm width × 10 μm height × 4 μm interspacing) are imprinted by both electrochemical and chemical (HF-H2O2) contact etching. A complete pattern transfer of pyramids is achieved with coarse nanoporous gold in both contact etching modes, at a rate of ~0.35 μm min−1. Under the same etching conditions, U-shaped line were only partially imprinted. The surface state after imprinting presents various defects such as craters, pores or porous silicon. Small walls are sometimes obtained due to imprinting of the details of the coarse gold nanostructure. We establish that np-Au electrodes can be turned into “np-Pt” electrodes by simply sputtering a thin platinum layer (5 nm) on the etching (catalytic) side of the electrode. Imprinting with np Au/Pt slightly improves the pattern transfer resolution. 2D numerical simulations of the valence band modulation at the Au/Si/electrolyte interfaces are carried out to explain the localized aspect of contact etching of n-type silicon with gold and platinum and the different surface state obtained after patterning. They show that n-type silicon in contact with gold or platinum is in inversion regime, with holes under the metal (within 3 nm). Etching under moderate anodic polarization corresponds to a quasi 2D hole transfer over a few nanometers in the inversion layer between adjacent metal and electrolyte contacts and is therefore very localized around metal contacts.

High-selectively determination of nitric oxide on nanoporous gold electrode

Here, a nanoporous gold electrode (NAu) was reported with a unique cone-shape nanohole structure for electrochemical sensing of nitric oxide (NO), which was fabricated via a facile sputtering technique on aluminum oxide membrane. Two kinds of nanoporous gold electrodes fabricated on different aluminum oxide membranes with aperture-size of 20 nm (2020NAu) and 200 nm (2040NAu) were obtained and compared in electrochemically active surface area and electro-oxidation activity. Concerning the determination of NO, it exhibited higher selectivity to NO2− and larger electro-oxidation current on 2020NAu electrode than those on 2040NAu electrode when their backsides were used as sensing interfaces. Meanwhile, the anti-interfering ability of bare 2020NAu electrode was also compared with that on Nafion-modified 2020NAu electrode. Results showed that common electroactive interferents such as H2O2, ascorbic acid, and uric acid could be hindered by cone-shape nanohole structure in the backside of 2020NAu electrode. On the basis of cyclic voltammetry, the linear range was from 4.75 × 10−8 to 9.50 × 10−7 M for NO sensing on 2020NAu electrode.

Fully Stretchable Capillary Microfluidics-Integrated Nanoporous Gold Electrochemical Sensor for Wearable Continuous Glucose Monitoring.

Biosensor systems for wearable continuous monitoring are desired to be developed into conformal patch platforms. However, developing such patches is very challenging owing to the difficulty of imparting materials and components with both high stretchability and high performance. Herein, we report a fully stretchable microfluidics-integrated glucose sensor patch comprised of an omnidirectionally stretchable nanoporous gold (NPG) electrochemical biosensor and a stretchable passive microfluidic device. A highly electrocatalytic NPG electrode was formed on a stress-absorbing 3D micropatterned polydimethylsiloxane (PDMS) substrate to confer mechanical stretchability, high sensitivity, and durability in non-enzymatic glucose detection. A thin, stretchable, and tough microfluidic device was made by embedding stretchable cotton fabric as a capillary into a thin polyurethane nanofiber-reinforced PDMS channel, enabling collection and passive, accurate delivery of sweat from skin to the electrode surface, with excellent replacement capability. The integrated glucose sensor patch demonstrated excellent ability to continuously and accurately monitor the sweat glucose level.

Nanoporous gold electrode prepared from two-step square wave voltammetry (SWV) and its application for electrochemical DNA biosensing of lung resistance related protein (LRP) gene

In this work, considering with the existing inherent deficiencies in the preparation of current porous gold electrode, uniform and ordered nanoporous gold (NPG) electrode prepared from two-step square wave voltammetry (SWV) was developed. The formation process, morphology and electrochemical properties were carefully characterized. Compared with naked gold electrode, NPG electrode illustrated the characteristics of improved surface area, enhanced signal response and repeatable preparation. Using NPG electrode as active surface, a new electrochemical DNA biosensor was established for the detection of lung resistance related protein (LRP) gene combined with thionine as an electroactive indicator. Likewise, the electrochemical behavior of the interaction between thionine and DNA molecular could be improved on this NPG electrode to enhance the detection performance of LRP gene. In a wide range from 200 f. to 7.5nM, the logarithm concentration of LRP gene was linear with the reduction peak current of thionine, and a detection limit of 60 f. was obtained. As a result, it had provided a good basis of experimental research for LRP gene detection. Meanwhile, this technique could be expected to be applied into the real analysis of LRP gene in clinical peripheral blood samples.

Nanoporous gold chemically de-alloyed from Au-based amorphous thin film for electrochemical nonenzymatic H2O2 sensing

Abstract Au-based amorphous thin films prepared by magnetron sputtering were used to fabricate nanoporous gold by chemical de-alloying obtaining three dimensional bicontinuous ligament-nanopore structure. The surface topography and microstructure of the nanoporous thin films were monitored after varying the de-alloying conditions such as de-alloying time and electrolyte concentration. The as-prepared nanoporous gold electrodes were investigated for H2O2 sensing by CV curves and chronoamperometry techniques. The results demonstrated the nanoporous gold thin film exhibited high sensitivity towards H2O2 sensing under the applied potential of −0.45 V versus SCE, suggesting their use as H2O2 sensor.

Oxygen Evolution Reaction on Nanoporous Gold Modified with Ir and Pt: Synergistic Electrocatalysis between Structure and Composition

AbstractActive oxygen evolution reaction electrocatalysts for water splitting have received great attention because of their importance in the utilization of renewable energy sources. Here, the electrochemical oxygen evolution reaction activities of a nanoporous gold (NPG)‐based electrode in acidic media are investigated. The dependence of the oxygen evolution reaction activity on the NPG surface area shows that the large electrochemical surface areas of the NPG are effectively utilized to enhance electrocatalytic activity. The NPG surfaces are modified with Pt using atomic layer electrodeposition methods, and the resulting NPG@Pt exhibited enhanced electrocatalytic activities compared to those of the NPG and flat Pt electrodes. Ir‐modified NPG (NPG@Ir) electrodes are prepared by spontaneous exchange of Ir on NPG surfaces and exhibit enhanced electrocatalytic activity compared to that of flat Ir surfaces. The modification of NPG@Pt with Ir results in NPG@Pt/Ir electrodes, and their electrocatalytic activities exceed those of NPG@Ir. The enhanced oxygen evolution reaction activity on NPG@Pt/Ir over that on NPG@Ir surfaces is examined by X‐ray photoelectron spectroscopy. The oxygen evolution reaction activity on NPG@Pt/Ir surfaces demonstrates synergistic electrocatalysis between the nanoporous surface structure and active electrocatalytic components.

Electrochemical Determination of As(III) at Nanoporous Gold Electrodes with Controlled Surface Area

Because arsenic (As) is a chemical substance toxic to humans, there have been extensive investigations on the development of As detection methods. In this study, the electrochemical determination of As on nanoporous gold (NPG) electrodes was investigated using anodic stripping voltammetry. The electrochemical surface area of the NPG electrodes was controlled by changing the reaction times during the anodization of Au for NPG preparation, and its effect on the electrochemical behavior during As detection was examined. The detection efficiency of the NPG electrodes improved as the roughness factor of the NPG electrodes increased up to around 100. A further increase in the surface area of the NPG electrodes resulted in a decrease of the detection efficiency due to high background current levels. The most efficient As detection efficiency was obtained on the NPG electrodes prepared with an anodization time of 50 s. The effects of the detection parameters and of the Cu interference in As detection were investigated and the NPG electrode was compared to flat Au electrodes.

Fabrication and Characterization of Nanoporous Gold Electrodes for Sensor Applications

Nanoporous gold electrodes are fabricated by etching white gold leaf in nitric acid. Samples of various thickness and alloy composition are used and etch time is varied. Electrode pore structure is characterized by scanning electron microscopy. Significant coarsening of ligament and pore structure is observed as etching time is varied from 5 to 90 min. Nanoporous gold electrode impedance is determined via electrochemical impedance spectroscopy. Test cells are constructed for this purpose consisting of two electrodes and an electrolyte. Both water and dichloroethane are examined as electrolyte solvents. A constant phase element with exponent greater than 0.9 is shown to provide an excellent description of interface impedance behavior for these test cells. Optimization is used to determine pseudo‐capacitance and exponent in each case. A comparison of nanoporous gold electrode impedance to planar gold electrode impedance is made to derive the effective area of the nanoporous gold electrode. It is found in all cases that interfacial impedances are much lower (>40 ×) than those of conventional electrodes. Test results show that significant variations in effective area can occur depending on the electrolyte employed.

High-Linearity Hydrogen Peroxide Sensor Based on Nanoporous Gold Electrode

Fast and accurate measurement of hydrogen peroxide is essential to glucose monitoring. In this work, we describe a highly efficient H2O2 sensor through the modification of Prussian blue (PB) on the nanoporous gold film (NPG). By de-alloying from Au-Ag film, we prepare the NPG film and characterize it through SEM and AFM. Tests using chronoamperometry and cyclic voltammetry on the Prussian blue modified NPG electrodes show an excellent electrochemical performance of the sensors. Owing to the comparatively huge specific surface area in the nanoporous structure, such electrodes significantly enhance the direct electron transfer from PB nanocrystals to NPG film. Comparing to earlier PB modified electrodes, the sensor fabricated in this work exhibits the widest linearity of 0.001∼17 mM, and produces a high selectivity of 708 μA·cm−2·mM−1.

A nanoporous gold-based electrochemical aptasensor for sensitive detection of cocaine.

The increasing application of aptamers in bioassays has triggered a lot of research interest for development of highly sensitive and selective sensing platforms. Herein, we report on the design of a sensitive cocaine biosensor by immobilizing the 5′-disulfide-functionalized end of an aptamer sequence on a nanoporous gold (NPG) electrode followed by the conjugation of its 3′-amino-functionalized end to 2,5-dihydroxybenzoic acid (DHBA) as the redox probe. In the presence of cocaine, the aptamer undergoes a conformational change from an open unfolded state to a closed conformation, which reduces the distance between DHBA and the electrode surface, resulting in the enhanced electron-transfer efficiency. Using square wave voltammetric method and under the optimal conditions, the cocaine aptasensor presented two linear responses in the concentration ranges between 0.05–1 and 1–35 μM, with an excellent detection limit of 21 nM. The proposed aptasensor provides a simple and low-cost method for cocaine detection with good reproducibility and accuracy. Furthermore, it could be regarded as a general model to investigate the unique function of aptamer-functionalized nanostructured electrodes to stablish highly advanced electrochemical biosensors for various target analytes of diagnostic importance.

Redox Potential Measurements in Red Blood Cell Packets Using Nanoporous Gold Electrodes

The redox potential of packed red blood cells (RBCs) was measured over a 56-day storage period using a newly developed potentiometric methodology consisting of a nanoporous gold electrode and a silver chloride coated silver reference electrode. Both milliliter- and microliter-sized volumes were separately evaluated. The addition of Vitamin C (VitC) in differing doses to the packed RBCs was also assessed as a means to improve redox stability and prolong storage duration. For RBCs containing only saline, the open-circuit potential (OCP) was ∼ -80 mV vs Ag/AgCl and drifted slightly with time; greater differences were also noted between different electrodes. The addition of exogenous VitC to the RBC shifts the OCP to more negative values, stabilizes the redox potential, and improves reproducibly between different electrodes due to the poising of blood. Over the 56-day storage period, the redox potential of the RBCs increased slightly, which can be attributed to change in pH and/or increasing oxidative stress during storage. Cyclic voltammograms acquired after open-circuit potential measurements showed a characteristic peak attributed to the oxidation of VitC. This peak decreased during storage with a time constant of 20.8 days. Likewise, the intercellular concentration of VitC increased with a time constant of 20.2 days as measured using a fluorescence assay. Collectively, these results demonstrate the usefulness of electrochemical measurements in the study of stored blood products.

A Microfluidic Platform to Study Astrocyte Adhesion on Nanoporous Gold Thin Films.

Nanoporous gold (np-Au) electrode coatings have shown improved neural electrophysiological recording fidelity in vitro, in part due to reduced surface coverage by astrocytes. This reduction in astrocytic spreading has been attributed to the influence of electrode nanostructure on focal adhesion (FA) formation. This study describes the development and use of a microfluidic flow cell for imposing controllable hydrodynamic shear on astrocytes cultured on gold surfaces of different morphologies, in order to study the influence of nanostructure on astrocyte adhesion strength as a function of np-Au electrode morphology. Astrocyte detachment (a surrogate for adhesion strength) monotonically increased as feature size was reduced from planar surfaces to np-Au, demonstrating that adhesion strength is dependent on nanostructure. Putative mechanisms responsible for this nanostructure-driven detachment phenomenon are also discussed.

Electrically Guided DNA Immobilization and Multiplexed DNA Detection with Nanoporous Gold Electrodes.

Molecular diagnostics have significantly advanced the early detection of diseases, where the electrochemical sensing of biomarkers (e.g., DNA, RNA, proteins) using multiple electrode arrays (MEAs) has shown considerable promise. Nanostructuring the electrode surface results in higher surface coverage of capture probes and more favorable orientation, as well as transport phenomena unique to nanoscale, ultimately leading to enhanced sensor performance. The central goal of this study is to investigate the influence of electrode nanostructure on electrically-guided immobilization of DNA probes for nucleic acid detection in a multiplexed format. To that end, we used nanoporous gold (np-Au) electrodes that reduced the limit of detection (LOD) for DNA targets by two orders of magnitude compared to their planar counterparts, where the LOD was further improved by an additional order of magnitude after reducing the electrode diameter. The reduced electrode diameter also made it possible to create a np-Au MEA encapsulated in a microfluidic channel. The electro-grafting reduced the necessary incubation time to immobilize DNA probes into the porous electrodes down to 10 min (25-fold reduction compared to passive immobilization) and allowed for grafting a different DNA probe sequence onto each electrode in the array. The resulting platform was successfully used for the multiplexed detection of three different biomarker genes relevant to breast cancer diagnosis.