Nanoporous Gold Electrode Research Articles

Electrochemical Detection of 4-Nitrophenol on a Nanoporous Gold Electrode Prepared by One-Step Anode Potential Step Method

The wastewater containing nitrophenol compounds is discharged into the water and soil environment, causing severe environmental pollution problems. In this paper, a one-step anodic potential step method involving Au(I) disproportionation was used to prepare nanoporous gold (D-NPG) electrodes for the electrochemical detection of 4-nitrophenol (4-NP) by differential pulse voltammetry (DPV). The D-NPG electrode has a high current response to the oxidation peak of 4-hydroxylaminophenol, a product of 4-NP electroreduction. This peak is used in the 4-NP detection. Under the optimized conditions, the resulting detection range is wide (0.01~20 µM) and the detection limit is low (3.5 nM), and its anti-interference ability is good. It can be used for the electrochemical detection of 4-NP in real water samples.

Mass Transport in Nanoporous Gold and Correlation with Surface Pores for EC1 Mechanism: Case of Ascorbic Acid

AbstractThe present work correlates the mass transport of ascorbic acid (AA), which undergoes an electrochemical reaction accompanied by a first‐order chemical reaction (EC1 mechanism), with the size of surface pores of nanoporous gold (NPG) films. NPG films were electrodeposited on gold microelectrodes, generating polycrystalline, and highly porous surfaces. Studies of AA electrooxidation reveal mainly diffusional mass transport driven by EC1 pathways on the NPG electrodes prepared by changing the deposition potential. Similar studies on the NPGs prepared by changing the deposition time exhibit mainly diffusional transport at shorter deposition times, and largely an adsorption process driven by EC1 at longer deposition times. Such transition in the reaction pathways from diffusion EC1 to adsorption EC1 is correlated with the evolution of micrometer large surface pores in NPG films prepared at longer deposition times, allowing the diffusion of AA from the bulk solution to the volume of NPG, which enhances the adsorption probability.

Effect of the residual silver and adsorbed lead anions towards the electrocatalytic methanol oxidation on nanoporous gold in alkaline media

Nanoporous gold (NPG) obtained by potentiostatic dealloying of an AuAg master alloy was pulverized, filled into a cavity microelectrode and its surface electrochemistry was investigated in NaOH and KOH solutions. This method yielded highly resolved undistorted voltammograms of this polycrystalline material with clear evidences for surface segregation of Ag during storage in air. One cycle in 0.1 M H2SO4 can remove most of the residual Ag from the surface as evidenced by voltammetry after back transfer to alkaline solution and by X-ray photoelectron spectroscopy (XPS). After removal of residual Ag from the surface, the surface undergoes rephasing forming wider {111} and {110} terraces as evidenced by Pb underpotential deposition (UPD). By adjustment of the bulk concentration of PbII species, a selective partial coverage of the high internal surface area of NPG could be intentionally adjusted. Lead species remain attached to the NPG surface at potentials positive of the UPD regions either as plumbates or PbIV species as evidenced by XPS. Those tools enable to disentangle effects on the electrocatalysis of the methanol oxidation reaction (MOR) in different potential regions for 1 M methanol in different concentrations of NaOH and KOH. The MOR commences at lower potentials in the presence of residual Ag. At very high potential, the presence of Ag species promotes the oxidation to CO2/CO32-. Tafel analysis after selective blocking of specific facets shows that the {110} terraces promote especially the first step of MOR while the {111} terraces enhance the rate of further steps that are rate-limiting at higher potentials. While high coverages by Pb UPD layers or adsorbed plumbate anions inhibit electrooxidation, the behavior of NPG electrodes with low coverages of Pb species is reminiscent to NPG electrodes with residual Ag. This could be caused by promoting the methoxide and/or OH- adsorption at low overpotentials and catalyzing complete oxidation by a surface bound PbIV species at higher potentials. The catalytic currents increase with base concentration and are higher for KOH compared to NaOH. Comparable effects of base concentration on the parameters of the Tafel lines suggest that this effect is a result of slightly stronger deprotonation of methanol in bulk KOH solutions.

Nanoporous Gold for Energy Applications.

Research activities using nanoporous gold (NPG) were reviewed in the field of energy applications in three categories: fuel cells, supercapacitors, and batteries. First, applications to fuel cells are reviewed with the subsections of proof-of-concept studies, studies on fuel oxidations at anode, and studies on oxygen reduction reactions at cathode. Second, applications to supercapacitors are reviewed from research activities on active materials/NPG composites to demonstrations of all-solid-state flexible supercapacitors using NPG electrodes. Third, research activities using NPG for battery applications are reviewed, mainly about fundamental studies on Li-air and Na-air batteries and some model studies on improving Li ion battery anodes. Although NPG based studies are the main subject of this review, some of meaningful studies using nanoporous metals are also discussed where relevant. Finally, summary and future outlook are given based on the survey on the research activities.

A facile approach for fabrication of three-dimensional platinum-nanoporous gold film and its application for sensitive detection of microRNA-126 combining with catalytic hairpin assembly reaction

MicroRNAs (miRNAs) as important biomarkers are closely related to cancers. Herein, a easy-facilitative nanocomposite which consists of Pt nanoparticles (PtNPs) and nanoporous gold (NPG) structure has been successfully prepared on Au electrode through a facile two-step square wave voltammetry (SWV) combining with electrodeposition method. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and cyclic voltammetry (CV) were used for microscopic analysis and electrochemical studies of the PtNPs/NPG modified electrode. Combined with the well-designed catalytic hairpin assembly (CHA) amplification strategy, the small amount of Pt particles presented over the NPG film electrode substantially improved the electrocatalytic activity in the determination of microRNA-126 (miRNA-126), and a significant increase of amperometric current response accompanied with low background signal could be monitored at PtNPs/NPG electrode compared to those of bare Au and NPG electrodes. Under optimal conditions, a wide linear range of 0.5–500 pM for the amperometric assay of miRNA-126 was achieved along with a limit of detection (S/N = 3) of 0.17 pM. We further applied this assay to detect target miRNA in the total RNA extract of H1975 cells. Therefore, the proposed biosensing platform exhibits good analytical performance such as facile, ultrasensitive, low-cost, and high anti-interference capability, which promises to be a potential and beneficial tool for biomedical research and early clinical diagnosis of cancers through detection of multiple miRNA biomarkers.

Surface-confined redox-active monolayers of a multifunctional anthraquinone derivative on nanoporous and single-crystal gold electrodes

We have investigated self-assembled molecular monolayers (SAMs) of a novel thiol-bearing anthraquinone derivative AQ(OMe) (1-amino-4-((3-mercapto-5-(methoxycarbonyl)phenyl)amino)-9,10-dioxo-9,10-dihydroanthracene-2-sulfonic acid) with multifarious protonation sites, on nanoporous gold (NPG) electrode surfaces. We employ cyclic voltammetry, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy (XPS). Single-crystal Au(111)-electrode surfaces area used as a reference. XPS exhibits S2p3/2 peaks at 161.1 and 162.1 eV, verifying well-defined AQ(OMe) Au-S bound SAMs. The mid-point potential (E1/2) of the SAMs shifts negatively with increasing pH with a slope of −55 mV pH−1, indicative of a 2e2H redox reaction. The anthraquinone group exhibits quasi-reversible behavior over a wide range of pH values (from2.8to9.0) and a sigmoidal electron transfer (ET) rate constant (kapp) with a maximum value at pH 5.5, which is likely to be close to the isoelectric point. The electrochemically addressable surface coverage, determined from the voltammetric peak areas, decreases with increasing pH. The apparent surface coverage on NPG is more than 13 times higher than that on Au(111), attributed mainly to the larger surface area and different structural packing modes, and possibly also to favorable terrace edges on NPG, but – somewhat unexpectedly – with little effect on kapp. The new anthraquinone derivative offers perspectives for gentle immobilization and efficient mediated interfacial electrochemical ET of redox proteins and other complex biomolecules.

Direct and Broadband Plasmonic Charge Transfer to Enhance Water Oxidation on a Gold Electrode.

Plasmonic photocatalysis via hot charge carriers suffers from their short lifetime compared with the sluggish kinetics of most reactions. To increase lifetime, adsorbates on the surface of a plasmonic metal may create preferential states for electrons to be excited from. We demonstrate this effect with O adsorbates on a nanoporous gold electrode. Nanoporous gold is used to obtain a broadband optical response, to increase the obtained photocurrent, and to provide a SERS-active substrate. Only with adsorbates present, we observe significant photocurrents. Illumination also increases the adsorbate coverage above its dark potential-dependent equilibrium, as derived from a two-laser in situ SERS approach. Density functional theory calculations confirm the appearance of excitable states below the Fermi level. The photocurrent enhancement and broadband characteristics reveal the potential of the plasmonic approach to improve the efficiency of photoelectrochemical water splitting.

Nanoporous gold electrodes modified with self-assembled monolayers for electrochemical control of the surface charge.

The electrochemical behaviour of nanoporous gold modified with self-assembled monolayers is investigated with regard to its point of zero charge (pzc) and proton transfer reaction. Due to their high surface-to-volume ratio and conductivity, nanoporous electrodes represent promising materials for numerous applications, including the immobilization of biomolecules in biotechnology and biosensing. Therefore, the fundamental understanding and controllability of the surface state of the electrode is essential. To achieve a precise surface charge control, nanoporous gold (npAu) is modified with self-assembled monolayers (SAMs) of different lengths (3-mercaptopropionic acid (MPA) and 16-mercaptohexadecanoic acid (MHDA)). Cyclic voltammetry and impedance spectroscopy are used to determine the pzc. The most distinct pzc, and thus the most precise charge control, is found for the long-chain MHDA. Subsequently, the proton transfer reaction was investigated as a function of pH and scan rate. The observed protonation/deprotonation reaction was qualitatively well in line with the literature for planar gold electrodes, albeit the fraction of electrochemical controllable SAMs increased by a factor of 10 compared to planar electrodes indicating attractive application potential.

An ultrasensitive electrochemical sensor for the detection of acetaminophen via a three-dimensional hierarchical nanoporous gold wire electrode.

Acetaminophen is one of the most commonly used non-steroidal anti-inflammatory drugs worldwide. However, due to the increasing popularity of this drug, overdosing and the contamination of ambient waterways have emerged as major issues. Here, we report on a reliable, ultrasensitive, and easy-to-use sensor for the electrochemical detection of acetaminophen. This sensor employs a gold wire electrode with a unique three-dimensional hierachical nanoporous structure, fabricated using a dissolution, disproportion and deposition procedure. In consideration of optimal sensitivity and reproducibility, the most suitable nanoporous gold electrode was employed for the detection of acetamiophen among a set of nanoporous electrodes made under different reaction times. It was found that the pore size, film thickness, and electrochemically active surface area (ECSA) played major roles in the fouling resistance of the developed sensor. The ECSA of the selected sensor was increased by 15.8 times after the post-treatment. The 3D nanoporous electrode demonstrated excellent performance for the detection of acetaminophen with a low detection limit of 3.37 nM, and a strong anti-interference capability. The developed nanoporous Au electrode proved effective for the detection of acetaminophen in real sheep serum, which confirmed its promising application for medical diagnostics and pollutant surveilliance in source waters.

Potentiometric Biosensing of Ascorbic Acid, Uric Acid, and Cysteine in Microliter Volumes Using Miniaturized Nanoporous Gold Electrodes.

Potentiometric redox sensing is a relatively inexpensive and passive approach to evaluate the overall redox state of complex biological and environmental solutions. The ability to make such measurements in ultra-small volumes using high surface area, nanoporous electrodes is of particular importance as such electrodes can improve the rates of electron transfer and reduce the effects of biofouling on the electrochemical signal. This work focuses on the fabrication of miniaturized nanoporous gold (NPG) electrodes with a high surface area and a small footprint for the potentiometric redox sensing of three biologically relevant redox molecules (ascorbic acid, uric acid, and cysteine) in microliter volumes. The NPG electrodes were inexpensively made by attaching a nanoporous gold leaf prepared by dealloying 12K gold in nitric acid to a modified glass capillary (1.5 mm id) and establishing an electrode connection with copper tape. The surface area of the electrodes was ~1.5 cm2, providing a roughness factor of ~16 relative to the geometric area of 0.09 cm2. Scanning electron microscopy confirmed the nanoporous framework. A linear dependence between the open-circuit potential (OCP) and the logarithm of concentration (e.g., Nernstian-like behavior) was obtained for all three redox molecules in 100 μL buffered solutions. As a first step towards understanding a real system, the response associated with changing the concentration of one redox species in the presence of the other two was examined. These results show that at NPG, the redox potential of a solution containing biologically relevant concentrations of ascorbic acid, uric acid, and cysteine is strongly influenced by ascorbic acid. Such information is important for the measurement of redox potentials in complex biological solutions.

Direct Electrochemical Enzyme Electron Transfer on Electrodes Modified by Self-Assembled Molecular Monolayers

Self-assembled molecular monolayers (SAMs) have long been recognized as crucial “bridges” between redox enzymes and solid electrode surfaces, on which the enzymes undergo direct electron transfer (DET)—for example, in enzymatic biofuel cells (EBFCs) and biosensors. SAMs possess a wide range of terminal groups that enable productive enzyme adsorption and fine-tuning in favorable orientations on the electrode. The tunneling distance and SAM chain length, and the contacting terminal SAM groups, are the most significant controlling factors in DET-type bioelectrocatalysis. In particular, SAM-modified nanostructured electrode materials have recently been extensively explored to improve the catalytic activity and stability of redox proteins immobilized on electrochemical surfaces. In this report, we present an overview of recent investigations of electrochemical enzyme DET processes on SAMs with a focus on single-crystal and nanoporous gold electrodes. Specifically, we consider the preparation and characterization methods of SAMs, as well as SAM applications in promoting interfacial electrochemical electron transfer of redox proteins and enzymes. The strategic selection of SAMs to accord with the properties of the core redox protein/enzymes is also highlighted.

Electrochemical aptasensor of bisphenol A constructed based on 3D mesoporous structural SBA-15-Met with a thin layer of gold nanoparticles

Herein, a simple and sensitive electrochemical aptasensor was fabricated on the electrodeposited nanoporous gold electrode. A layer of gold nanoparticle deposited on the surface of 3D mesoporous structures of SBA-15-Met provided a clean, stable, and biocompatible surface for self-assemble of chain BPA aptamers that bring about considerable advantages in both molecular recognition and signal enhancement. The most important factors affecting the design of the aptasensor, such as aptamer concentration, aptamer incubation time, BPA incubating time, and the pH of the solution, were optimized using a multivariate experiment design. For this purpose, a five-level central composite design and a response surface methodology were used. Under the optimized conditions, the proposed aptasensor exhibited a wide linear range from 10 to 1200 pM with an ultralow detection limit of 3.65 pM for BPA. According to molecular docking computational, some hydrogen bonds and the π-π interactions have led to a strong affinity between the aptamer and BPA molecules which causes the developed aptasensor to have high sensitivity and selectivity for BPA determination in food samples.

Benzene Diazonium Sulfonate Modified Nanoporous Gold Electrodes for the Direct Detection of Copper(II) Ions

AbstractThe detection of Cu2+on surface modified nanoporous gold in aqueous solution is reported. Nanoporous gold electrodes were prepared with a wide range of pore sizes by dealloying of sputtered gold–silver alloys in concentrated nitric acid. Varying the pore sizes can significantly improve the electroactive surface area of the electrodes as well as protect the surface from biofouling materials, both necessary for the direct detection of trace metals in untreated samples. Two surface modificationsi. e. a self‐assembled monolayer of 3‐mercapto‐1‐propanesulfonate and electro‐grafted benzene diazonium sulfonate were evaluated. Underpotential deposition of Cu2+and subsequent stripping of the deposited copper was achieved utilizing square wave voltammetry. The sensor was capable of accurately measuring Cu2+concentrations in a range of water samples (tap, rain and river water) without the necessity of prior pre‐treatment of the samples. The sensor had a linear range of 0.2 to 25 μM, which encompasses the accepted maximum limit for the concentration of copper in drinking water (20.5 μM). The sensitivity was 8.18 μA cm−2 μM−1with a limit of detection of 18.9 nM (∼1.2 ppb). The response of the sensor in artificial human serum was unaffected by the sample matrix, demonstrating the ability of the system to resist fouling by serum

Electrochemical Oxidation of Monosaccharides at Nanoporous Gold with Controlled Atomic Surface Orientation and Non-Enzymatic Galactose Sensing.

Non-enzymatic saccharide sensors are of great interest in diagnostics, but their non-selectivity limits their practical diagnostic abilities. In this study, we investigated the electrochemical oxidation of monosaccharides at nanoporous gold (NPG) catalysts with different contributions of surface crystallographic orientations. Fructose elicited no clear electrochemical response, but glucose, galactose, and mannose produced clear oxidative current. The onset potentials for oxidation of these saccharides depended on the surface atomic structure of the NPG. The oxidation potential was approximately 100 mV less positive at the Au(100)-enhanced NPG than at the Au(111)-enhanced NPG. Furthermore, the voltammetric responses significantly differed among the saccharides. Galactose was oxidized at less positive potential and exhibited a higher current response than the other saccharides. This tendency was enhanced in the presence of chloride ions. These features enabled the selective and sensitive detection of galactose at an NPG electrode without enzymes under physiological conditions. A linear range of 10 μM to 1.8 mM was obtained in the calibration plot, which was comparable to those in previously reported enzymatic galactose sensors. Thus, we demonstrated that controlling the crystallographic orientation on the nanostructured electrode surface is useful in developing electrochemical sensors.

A high-performance amperometric sensor based on amonodisperse Pt-Au bimetallic nanoporous electrode for determination of hydrogen peroxide released from living cells.

A neotype electrochemical sensor based on a three-dimensional nanoporous gold (3D-NPG) electrode decorated with ultra-thin platinum nanoparticles (Pt NPs) was fabricated for high-performance electrocatalysis and sensitive determination of hydrogen peroxide (H2O2) released from pheochromocytoma (PC12) cells. The monodisperse Pt-Au bimetallic nanoporous (Pt-Au-BNP) electrode prepared by cyclic voltammetry electrodepositing monolayer Pt NPs on the surface ofthe 3D-NPG electrode was characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive spectroscopy (EDS). Amperometric response for H2O2 measurement was chosen at an applied potential of - 0.4V. Upon optimal conditions, the wide linear range for the amperometric determination of H2O2 was from 0.05μM to 7.37mM, with a limitof detection (S/N = 3) of 1.5 × 10-8mol/L and a high sensitivity of 1.125μAμM-1cm-2, certifying the largeelectrocatalytic actionof the Pt-Au-BNP electrode. Theproposed sensor has been successfully applied tothe dynamic determination of H2O2 released from PC12 cells (from which the H2O2 generated by each cell was about 52.5amol) with negligible interference. Thus, the proposed new electrochemical sensor displays potential applications for the dynamic, real-time monitoring of key small molecules secreted by living cells, further deepening theunderstanding of cell behavior stimulated by foreign materials. Graphical Abstract .

Unraveling the reaction mechanism of co-reactant free in-situ cathodic solid state ECL of Ru(bpy)32+ molecule immobilized on Nafion coated nanoporous gold electrode

The electrochemiluminescence (ECL) of Ru(bpy)32+ molecule which is immobilized on Nafion coated nanoporous gold electrode (Ru(bpy)32+/Nafion/NPG) is investigated without involving any additional co-reactant in 0.1 M phosphate buffer solution (PBS) at pH 7.4 and the observed results are compared with Ru(bpy)32+ molecule immobilized on Nafion coated polycrystalline gold (Ru(bpy)32+/Nafion/pc-Au). Because of high surface area volume ratio and more active sites on NPG, the obtained co-reactant free unusual cathodic ECL signal at less cathodic potentials (0.1 V vs Ag/AgCl) showed 40 times higher intensity on Ru(bpy)32+/Nafion/NPG when compared with Ru(bpy)32+/Nafion/pc-Au in PBS (pH 7.4). Interestingly, this kind of ECL signal is not observed in the case of Ru(bpy)32+ molecule which is immobilized on Nafion coated glassy carbon (GC) and polycrystalline platinum (pc-Pt) electrodes. Based on the detailed experiments on effect of different pH, various direction of scanning potential and in-situ spectroelectrochemical studies, we proposed a new reaction mechanism for the unusual cathodic ECL on Ru(bpy)32+/Nafion/NPG. Upon electrochemical oxidation on Ru(bpy)32+/Nafion/NPG at higher potential, the stored Ru(bpy)32+ is converted into Ru(bpy)33+ at the same time generated reactive intermediate species like OH radical from the conversion of Au(OH)3 to Au0 during the reduction process leads energetic electron transfer reaction to proceed the excited state of Ru(bpy)32+ and then emits light. Unlike the conventional ECL system, this kind of co-reactant free in-situ generated solid state ECL approach offers to the scope in various bio analytical applications.

Facile control of surface crystallographic orientation of anodized nanoporous gold catalyst and its application for highly efficient hydrogen evolution reaction

Controlling the surface crystallographic orientation in a nanostructured catalyst is expected to be effective in improving its catalytic performance. Herein, a simple one-step method for producing an anodized nanoporous gold (NPG) electrode with a controlled contribution of atomic surface orientations is reported. Anodization of a conventional Au electrode in 35 and 500 mM HCl solutions produced nanoporous structures with a considerable amount of Au(111) and Au(100) surfaces, respectively. The former exhibited remarkably efficient electrocatalysis for the hydrogen evolution reaction (HER). The obtained results show that the present method is valuable for the development of effective catalysts for various applications utilizing nanoporous structures.

High-performance three-dimensional nanoporous gold based electrodes for flexible all-solid-state supercapacitors

High-performance, flexible reduced graphene oxide (rGO)-polypyrrole (PPy)/nanoporous gold (NPG) electrodes are prepared by a facile electrochemical interfacial polymerization method using NPG as the substrate. Owing to the synergistic effect of three-dimensional (3D) porous architecture and superior electrical conductivity, the prepared electrode not only exhibits a good areal capacitance (232 mF/cm2), but also shows high volumetric capacitance (555.6 F/cm3). Hence, an all-solid-state supercapacitor assembled with rGO-PPy/NPG electrodes exhibits a high energy density of 238.8 mW h/cm3. Besides, this flexible supercapacitor also shows excellent cycling stability with retention of 91% with respect to the initial capacitance after 2000 galvanostatic charge–discharge cycles. This work offers a novel avenue for designing flexible supercapacitor.

Redox State Control of Human Cytoglobin by Direct Electrochemical Method to Investigate Its Function in Molecular Basis

The direct electron transfer between human cytoglobin (Cygb) and the electrode surface, which would allow manipulating the oxidation states of the heme iron in Cygb, was first observed by immobilizing Cygb on a nanoporous gold (NPG) electrode via a carboxy-terminated alkanethiol. The voltammetric performances of the wild type and mutated Cygb-immobilized NPG electrodes were evaluated in the absence or presence of potential substrates. The obtained results demonstrated that the usefulness of the proposed method in understanding the function of Cygb in molecular basis.

Electrochemical Studies of Hydrogen Peroxide Oxidation on a Nanoporous Gold Surface: Fundamental and Analytical Applications

A nanoporous gold (NPG) surface with high electroactive surface area and roughness factor was prepared via anodization-electrochemical reduction method and was used as a new platform to study the electrochemical oxidation of hydrogen peroxide (H2O2). Field-emission scanning electron microscopy (FE-SEM) analysis, cyclic voltammetry, and amperometry techniques were employed for structural morphology and electrochemical studies. The H2O2 oxidation peak potential was significantly shifted (∼0.23 V) towards a less positive direction with increased current response at the NPG surface compared to that of a bare gold electrode. The mechanism for the H2O2 anodic oxidation at the NPG surface was proposed based on the calculated transfer coefficient (α = 0.32) and the total number of electrons involved (n = 2.0) in the electrochemical process. Besides, the diffusion coefficient (D) of H2O2 in PBS solution was also calculated using the Randles-Sevcik equation (2.18 × 10−5 cm2 s−1). In addition, the analytical application of the NPG electrode was demonstrated by amperometric detection of H2O2 at a relatively less positive potential (0.57 V). The current increased proportionally to H2O2 concentration in the linear range of 0.01 to 1.8 mM and the detection limit was found to be 0.6 μM (S/N = 3).