Au Nanoelectrodes Research Articles

Anodische H2O2‐Erzeugung in Elektrolyten auf Karbonatbasis – Mechanistische Erkenntnisse Mittels Elektrochemischer Rastermikroskopie

AbstractFür die anodische H2O2‐Bildung kann die Elektrolytzusammensetzung den Reaktionsweg in Richtung einer erhöhten Produktbildung lenken. Frühere Untersuchungen zur Optimierung der Elektrolytzusammensetzung haben gezeigt, dass die Auswirkungen des molaren Anteils der Karbonatspezies für verschiedene Anodenmaterialien unterschiedlich sind, und daher gibt es weiterhin Kontroversen über die Reaktionswege und die an der H2O2‐Bildung beteiligten Spezies. In Anbetracht der Tatsache, dass die Wasseroxidation zur Freisetzung von Protonen in der Anodenmikroumgebung führt, würde die entsprechende Ansäuerung eine Gleichgewichtsverschiebung zwischen den Karbonatspezies bewirken, was wiederum den Reaktionsweg modulieren könnte. Wir haben die Veränderungen im Anteil der Karbonatspezies in der Nähe einer arbeitenden Anode bestimmt, indem wir lokale pH‐Messungen mit einer Au‐Nanoelektrode, die mittels elektrochemischer Scherkraft‐Rastermikroskopie (SECM) in unmittelbarer Nähe einer sich in Betrieb befindlichen Anode positioniert war, durchgeführt haben. Es konnte bestätigt werden, dass die wichtigste anionische Spezies an der Grenzfläche HCO3− ist, und zwar bei Potentialen, bei denen H2O2 bevorzugt gebildet wird, unabhängig vom pH‐Wert im Volumen der Lösung. Die gleichzeitige Verwendung einer Au−Pt‐Doppelmikroelektrode in Generator‐Kollektor‐SECM‐Messungen zeigt, dass die lokale HCO3−Konzentration insgesamt durch den Oxidationsstrom, die Pufferkapazität und den pH‐Wert des Elektrolyten bestimmt wird.

Anodic H2O2 Generation in Carbonate-Based Electrolytes-Mechanistic Insight from Scanning Electrochemical Microscopy.

For the anodic H2O2 generation, it has been shown that the electrolyte composition can steer the reaction pathway toward increased H2O2 generation. Previous efforts made on composition optimization found that the impact of the molar fraction of carbonate species varies for different anodes, and therefore, controversies remain concerning the reaction pathways as well as the species involved in H2O2 formation. Considering that water oxidation results in the liberation of protons within the anode microenvironment, the corresponding acidification would cause an equilibrium shift between carbonate species, which in turn may modulate the reaction pathway. We determined the changes in the fraction of carbonate species in the vicinity of an anode by performing local pH measurements using a Au nanoelectrode positioned in close proximity to an operating anode by shear-force scanning electrochemical microscopy (SECM). It could be confirmed that the main anionic species at the interface is HCO3 -, at potentials where H2O2 is preferentially formed, regardless of the pH value in the bulk. The simultaneous use of a Au-Pt double barrel microelectrode in generator-collector SECM measurements demonstrates that the local HCO3 - concentration is collectively determined by the oxidation current, buffer capacity, and bulk pH of the electrolyte.

Au Micro‐ and Nanoelectrodes as Local Voltammetric pH Sensors During Oxygen Evolution at Electrocatalyst‐Modified Electrodes

The scarcity of state‐of‐the‐art oxygen evolution reaction (OER) electrocatalysts has led to intensive research on alternative viable electrocatalytic materials. While activity and cost are the main factors to be sought after, the catalyst stability under harsh acidic conditions is equally crucial. Considering that OER is a proton‐coupled electron‐transfer reaction that involves local acidification of the reaction environment by liberation of H+, the catalyst stability can be largely compromised in such conditions. Consequently, probing the pH value near the catalyst surface under operation leads to a deeper understanding of this process. The applicability of bare Au microelectrodes and nanoelectrodes as sensitive local pH probes during OER is shown in this work by using scanning electrochemical microscopy (SECM). Two case studies are presented, including the state‐of‐the‐art OER catalyst (IrO2) in acidic media and a ZnGa2O4 catalyst in alkaline buffered solution, demonstrating the suitability of the Au probe to accurately determine the local pH value in a wide pH range.

Self-Assembled Au Nanoelectrodes: Enabling Low-Threshold-Voltage HfO2-Based Artificial Neurons.

Filamentary-type resistive switching devices, such as conductive bridge random-access memory and valence change memory, have diverse applications in memory and neuromorphic computing. However, the randomness in filament formation poses challenges to device reliability and uniformity. To overcome this issue, various defect engineering methods have been explored, including doping, metal nanoparticle embedding, and extended defect utilization. In this study, we present a simple and effective approach using self-assembled uniform Au nanoelectrodes to controll filament formation in HfO2 resistive switching devices. By concentrating the electric field near the Au nanoelectrodes within the BaTiO3 matrix, we significantly enhanced the device stability and reduced the threshold voltage by up to 45% in HfO2-based artificial neurons compared to the control devices. The threshold voltage reduction is attributed to the uniformly distributed Au nanoelectrodes in the insulating matrix, as confirmed by COMSOL simulation. Our findings highlight the potential of nanostructure design for precise control of filamentary-type resistive switching devices.

Electrochemical and electrocatalytic performance of single Au@Pt/Au bimetallic nanoparticles

Bimetallic nanoparticles (NPs) are widely used in energy catalysis, chemical synthesis, and chemical/biosensing, but their performance at single NPs level is still needed to investigate. In this work, bimetallic Au@Pt/Au NPs with triple-layered core-shell structure was successfully prepared and characterized. The single bimetallic Au@Pt/Au NPs were immobilized on the surface of Au nanoelectrode through electrostatic adsorption. The results have showed that the single Au@Pt/Au NPs have size-dependent properties for voltametric response and electron-transfer rate, and exhibit excellent electrocatalytic activity, long-term tolerance and anti-toxicity compared with the traditional Pt catalyst. This work provides new insight into conventional electrocatalysis and will likely give a good platform for designing entirely new catalysts at single NPs level.

Hairpin DNA-based electrochemical amplification strategy for miRNA sensing by using single gold nanoelectrodes.

A new sensor has been developed to detect miRNA-15 using nanoelectrodes and a hairpin DNA-based electrochemical amplification technique. By utilizing a complex DNA cylinder connected with hairpin DNA1, the sensor is able to absorb more methylene blue (MB) than simple double-stranded DNA. Another hairpin DNA2 is modified on an Au nanoelectrode surface and, when miRNA-15 is introduced, it triggers a chain reaction. This reaction unlocks two hairpins alternatively to polymerize into a complex structure that attaches more MB. The miRNA-15 is then replaced by DNA1 due to strand displacement reactions and continues to react with the next DNA2 to achieve circular amplification. The electrochemical signal from MB oxidation has a linear relationship with the miRNA-15 concentrations, making it possible to detect miRNA-15. Moreover, this method can be readily adapted for the detection of various other miRNA species. The newly devised nanosensor holds promising applications for the in vivo detection of miRNA-15 within biological systems, which is achieved by leveraging the advantageous characteristics of nanoelectrodes, including their low resistance-capacitance time constant, rapid mass transfer kinetics, and small diameter.

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Electrofluorochromic imaging analysis of dopamine release from living PC12 cells with bipolar nanoelectrodes array

The coupling of bipolar electrode (BPE) arrays and electrofluorochromic (EFC) imaging has exhibited great abilities in bioanalysis. However, the imaging resolution and analytical performance are hampered by the large size of the electrode and the rapid diffusion of EFC molecules on the electrode surface. Here, to address the challenges, bipolar nanoelectrodes (BPnE) array and in situ immobilization strategy of EFC molecules were proposed. Anodized aluminum oxide (AAO) template-assisted Au nanoelectrodes array with high density was fabricated as BPnE array for high spatial imaging resolution. By electrically polymerizing EFC molecules on the surface of single Au nanoelectrode, the rapid diffusion of EFC molecules on the electrode surface was not only avoided, but also realizing electrofluorescent imaging on an individual nanoelectrode. Using dopamine (DA) released from living PC12 cells as a model, the proposed strategy exhibited an ultra-high sensitivity for DA analysis with a detection limit of 0.45 nmol/L and the DA release amount from a single cell was calculated to be 0.13 pmol/L. Moreover, the dynamic change of DA release under the drug stimulation from living PC12 cells could also be monitored.

Uniform arrays of gold nanoelectrodes with tuneable recess depth.

Nanoelectrode arrays are much in demand in electroanalytical chemistry, electrocatalysis, and bioelectrochemistry. One of the promising approaches for the preparation of such systems is templated electrodeposition. In the present study, porous anodic alumina templates are used to prepare Au nanoelectrode arrays. Multistage electrodeposition is proposed for the formation of recessed electrodes with the ability to tune the distance between the surface of the porous template and the top surface of the nanoelectrodes. A set of complementary techniques, including chronoamperometry, coulometry, and scanning electron microscopy, are used to characterize the nanoelectrode arrays. The number of active nanoelectrodes is experimentally measured. The pathways to further improve the recessed nanoelectrode arrays based on anodic alumina templates are discussed.

Elucidating the influence of anchoring geometry on the electronic transport of carbon-atom chains

In this work, the transport properties of carbon atom chains sandwiched between Au (100) nanoelectrodes with different anchored structures have been studied using a combination of first principles and nonequilibrium Green’s function. The results show that the transport properties of the carbon chain are not only related to the chain length and the number of C atoms (odd/even), but also related to the anchoring structure of nanoelectrodes. For the same chain length and bias voltage, the current of the H–H model is about three times that of the T–T model due to more charge transfer. Meanwhile, it was also found that the C atom in contact with the electrode has a large amount of charge transfer, which provides an effective way for the electronic doping of semiconductor devices. These findings can provide theoretical support for the design of molecular devices based on the C-chain.

Dual-signaling amplification strategy for glutathione sensing by using single gold nanoelectrodes

A new nanosensor for glutathione (GSH) detection by use of single nanoelectrodes has been developed through a dual-signaling ratiometric amplification strategy. Ag nanoparticles (Ag NPs) metalized DNA1 was modified on an Au nanoelectrode surface. Due to the strong affinity between Ag NP and GSH, Ag NPs could be removed by the addition of GSH. The remaining metalized DNA1 could hardly form a double strand, while the de-metalized DNA1 could hybrid with DNA2 and DNA3 to form a complex structure to adsorb methylene blue (MB), and then the electrochemical signal of differential pulse voltammetry (DPV) from MB oxidation could be observed. With the addition of GSH, the peak current of MB oxidation at about −0.27 V (IMB) increases, while the signal of Ag oxidation at about 0.1 V (IAg) decreases. It was found that there had a linear relationship between the ratio of dual-signal (IMB/IAg) and the GSH concentrations, which could be used to detect GSH. The ratiometric nanosensor is label-free, easy to operate, and can eliminate inherent system errors. Considering the advantages of nanoelectrodes, such as low IR drop, fast response, and small overall dimension, this developed nanosensor can be used for GSH detection living systems (e.g., cell lysate).

Probing the local activity of CO2 reduction on gold gas diffusion electrodes: effect of the catalyst loading and CO2 pressure.

Large scale CO2 electrolysis can be achieved using gas diffusion electrodes (GDEs), and is an essential step towards broader implementation of carbon capture and utilization strategies. Different variables are known to affect the performance of GDEs. Especially regarding the catalyst loading, there are diverging trends reported in terms of activity and selectivity, e.g. for CO2 reduction to CO. We have used shear–force based Au nanoelectrode positioning and scanning electrochemical microscopy (SECM) in the surface-generation tip collection mode to evaluate the activity of Au GDEs for CO2 reduction as a function of catalyst loading and CO2 back pressure. Using a Au nanoelectrode, we have locally measured the amount of CO produced along a catalyst loading gradient under operando conditions. We observed that an optimum local loading of catalyst is necessary to achieve high activities. However, this optimum is directly dependent on the CO2 back pressure. Our work does not only present a tool to evaluate the activity of GDEs locally, it also allows drawing a more precise picture regarding the effect of catalyst loading and CO2 back pressure on their performance.

Front Cover: Integration of Individual Functionalized Gold Nanoparticles into Nanoelectrode Configurations: Recent Advances (Eur. J. Inorg. Chem. 40/2020)

The Front Cover shows in an artistic manner a Janus-type gold nanoparticle surrounded by -COOH- and -NH2-bearing ligands. In the background is a set of Au and Pt nanoelectrodes in different colors. They enable directed immobilization of the Janus particles to achieve a diode-like electrical transport characteristic. Concepts for synthesis, integration, and nanoelectronic applications of gold nanoparticles are discussed. More information can be found in the Minireview by U. Simon et al.

Selective Electrochemical Determination of Dopamine with Molecularly Imprinted Poly(Carbazole‐co‐Aniline) Electrode Decorated with Gold Nanoparticles

AbstractDopamine (DA) is a significant neurotransmitter in the central nervous system, coexisting with uric acid (UA) and ascorbic acid (AA). UA and AA are easily oxidizable compounds having potentials close to that of DA for electrochemical analysis, resulting in overlapping voltammetric response. In this work, a novel molecularly imprinted (MI) electrochemical sensor was proposed for selective determination of DA (in the presence of up to 80‐fold excess of UA and AA), relying on gold nanoparticles (Aunano)‐decorated glassy carbon (GC) electrode coated with poly(carbazole (Cz)‐co‐aniline (ANI)) copolymer film incorporating DA as template (DA imprinted‐GC/P(Cz‐co‐ANI)‐Aunano electrode, DA‐MIP‐Aunano electrode). The DA recognizing sensor electrode showed great electroactivity for analyte oxidation in 0.2 mol L−1 pH 7 phosphate buffer. Square wave voltammetry (SWV) was performed within 10−4–10−5 mol L−1 of DA, of which the oxidation peak potential was observed at 0.16 V. The limit of detection (LOD) and limit of quantification (LOQ) were 2.0×10−6 and 6.7×10−6 mol L−1, respectively. Binary and ternary synthetic mixtures of DA‐UA, DA‐AA and DA‐UA‐AA yielded excellent recoveries for DA. Additionally, DA was quantitatively recovered from a real sample of bovine serum spiked with DA, and determined in concentrated dopamine injection solution. The developed SWV method was statistically validated against a literature potentiodynamic method using a caffeic acid modified‐GC electrode.

Spin induced inverter circuit with trinuclear chromium complexes

Density functional theory is used in conjunction with the non-equilibrium Green's functions approach to explore the spin polarized charge transport properties of homo-nuclear tri‑chromium and hetero-nuclear chromium‑ruthenium metal-string complexes. The metal strings comprise Cr-Cr-Cr, Cr-Cr-Ru, Cr-Ru-Cr, and Cr-Ru-Ru metal centers that are sandwiched between two Au (111) nano-electrodes. As deduced from the results, the Cr-Cr-Cr metal-string junctions could potentially be exploited in designing an inverter logic circuit to transfer the data at nanoscale. Considerably It also turns out that, among these single molecular junctions, Cr-Cr-Ru and Cr-Ru-Ru exhibit significant spin-up filtering with about 100% efficiency. More importantly, the Cr-Ru-Ru metal string complex demonstrates the negative differential resistance (NDR) effects in addition to its spin filtering properties. To analyze these phenomena, we examine the respective transmission spectra along with the potential drop across all the four metal-string junctions.

Single mercury nanoelectrode: Single nucleus growth on Au nanoelectrode and its sensing application

In this paper, single Hg nanoelectrodes were prepared by single nucleation/growth of mercury on the surface of single gold nanodisk electrodes, which were fabricated by improved laser-assisted pulling method. The nucleation/growth process was characterized by scanning electron microscopy (SEM), transition electron microscopy (TEM) and electrochemical method. Pb2+ and Cu2+ could be accumulated and melted with single Hg nanoelectrodes (amalgam formed) through preconcentration step, and then two typical anodic waves would be obtained through square-wave anodic stripping voltammetry (SWASV) method. The peak currents were corresponded to the concentration of Pb2+ and Cu2+, and thus the single Hg nanoelectrodes could be used as a kind of novel nanosensor for simultaneous detection of lead and copper with high sensitivity and selectivity.

Electrochemical Determination of TNT, DNT, RDX, and HMX with Gold Nanoparticles/Poly(Carbazole-Aniline) Film-Modified Glassy Carbon Sensor Electrodes Imprinted for Molecular Recognition of Nitroaromatics and Nitramines.

Since nitroaromatic- and nitramine-type energetic materials, mostly arising from military activities, are persistent pollutants in soil and groundwater, on-site sensing of these hazardous chemicals has gained importance. A novel electrochemical sensor was designed for detecting nitroaromatic- and nitramine-type energetic materials, relying on gold nanoparticles (Aunano), modified glassy carbon (GC) electrode coated with nitro-energetic memory-poly(carbazole-aniline) copolymer (Cz- co-ANI) film (e.g., TNT memory-GC/P(Cz- co-ANI)-Aunano modified electrode). Current was recorded against concentration to build the calibration curves that were found to be linear within the range of 100-1000 μg L-1 for 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT): 50-1000 μg L-1 for 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The corresponding limits of detection were 25 μg L-1 for TNT, 30 μg L-1 for DNT, and 10 μg L-1 for both RDX and HMX, using nitro-energetic memory-GC/P(Cz- co-ANI)-Aunano electrodes. These electrodes were used separately, and specific determinations were made in various mixtures of nitro-energetic materials. The developed method could be efficiently used in electroanalyzing nitroaromatics and nitramines in military explosives (i.e., comp B, octol, and comp A5). The sensor electrodes were specific for the tested nitro-energetic compounds and did not respond to paracetamol-caffeine-based analgesic drug, acetylsalicylic acid (aspirin), sweetener, and sugar that can be used as camouflage materials in passenger belongings. The developed method was statistically validated against the standard LC-MS reference method in contaminated clay soil samples containing TNT and RDX explosives.

Development of Glass‐sealed Gold Nanoelectrodes for in vivo Detection of Dopamine in Rat Brain

AbstractIn vivo monitoring of dopamine (DA) in cerebral nervous system with no traumatic brain injury is still a challenging work for reliably evaluating the role that DA plays in physical and cognitive functionality. In this work, to reduce the traumatic injury on the brain tissue, we develop a glass‐sealed Au nanoelectrode (GSNE) with nanoscale physical dimention for cerebral DA detection. First, an Au microwire was sealed into the pre‐thinned silica capillary. Then, the obtained Au/capillary assembly was pulled into two tips followed by polishing away the excess glass to expose the Au nanotip. The morphology of GSNE was characterized by scanning electron microscopy and transmission electron microscopy. The results showed that this nanoelectrode can be quickly fabricated with high reproducibility. By further electrodepositing cluster‐like gold nanostructure on GSNE and modifying Nafion, amperometric detection of DA with high sensitivity and good selectivity were achieved on the Nafion‐modified Au/GSNE. Moreover, the Nafion‐modified Au/GSNE possessed advantages of easy tissue insertion and minimum tissue disturbance. Thus, the developed electrochemical nanosensor was successfully used for amperometrically monitoring DA level in the striatum of anesthetic rats.

Penetrating Nanoelectrodes for Stimulation of Single-Cells

In this paper, we report on the development of two nanoelectrode geometries, namely nanopillar and nanofin to interface with live neuronal cells. Electrical stimulus is delivered via penetrating nanoelectrodes in order to stimulate live PC12 cells. The paper also reports on the time dynamics of cell penetration process using the nanoelectrode. We show that (a) nanoelectrodes of optimized size and shape can penetrate into PC12 cells without any loss of cellular function and (b) electrical stimulus can be applied through the penetrating electrodes in order to stimulate or differentiate PC12 cells without the use of conventional growth factors, genetic manipulators, cytokines or other chemical reagents. The nanoelectrodes (nanopillars) are designed to overcome the trade-offs between electrode impedance and electrode size. Compared to conventional 3x3 array of 150 nm diameter nanowire electrodes, the current 50-150 nm nanoelectrodes have reduced electrode impedance by factor of at least 20 due to a large capacitance and small charge transfer resistance, while maintaining minimal cell damage. The fabrication of nanoelectodes starts from silicon on insulator (SOI) substrate, followed by a series of lithographic patterning, deposition and etching steps including focused ion beam (FIB) ion-milling. The precise placement of cells on the nanoelectrodes is a challenging process. After surface functionalization with silane coupling (uncoated Si) or thiol chemistry (Au nanoelectrodes), non-contact microprinting method using an inkjet printer was also used to place cells on the nanoelectrodes. For cell differentiation, pulsed DC stimuli with monophasic 100 ms square pulse of 100 mV/cm field strength and 1 Hz frequency are applied. In order to validate this differentiation process, gene expression markers and morphological changes are monitored. Figure 1 shows the dynamics of the cell penetration process over time, recorded at 5 minutes to 5 hours. Figure 2 shows fixed SEM images of differentiated cells being penetrated by the nanoelectrodes without any loss of cellular function. Additionally, it is evident (figure 3) that the nanoelectrodes are clearly assessing the interior of the cells’ cytosol. This image was taken after FIB milling of one half of the electrode-cell interface. Figure 1

Nanofabrication of the gold scanning probe for the STM-SECM coupling system with nanoscale spatial resolution

Scanning probe is the key issue for the electrochemical scanning probe techniques (EC-SPM) such as EC-scanning tunnel microscopy (STM), EC-atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM), especially the insulative encapsulation of the nanoelectrode probe for both positioning and electrochemical feedbacks. To solve this problem, we develop a novel fabrication method of the gold nanoelectrodes: firstly, a micropipette with nanomter-sized orifice was prepared as the template by a laser puller; secondly, the inside wall of micropipette apex was blocked by compact and conic Au nano-piece through electroless plating; thirdly, the Au nano-piece was grown by bipolar electroplating and connected with a silver wire as a current collector. The fabricated Au nanoelectrode has very good voltammetric responses for the electrodic processes of both mass transfer and adsorption. The advantage lies in that it is well encapsulated by a thin glass sealing layer with a RG value lowered to 1.3, which makes it qualified in the SECM-STM coupling mode. On one hand, it can serve as STM tip for positioning which ensures the high spatial resolution; on the other hand, it is a high-quality nanoelectrode to explore the local chemical activity of the substrate. The nanofabrication method may promote the SPM techniques to obtain simultaneously the physical and chemical images with nanoscale spatial resolution, which opens a new approach to tip chemistry in electrochemical nanocatalysis and tip-enhanced spectroscopy.