Au Nanoelectrodes Research Articles

Toward Single-Enzyme Molecule Electrochemistry: [NiFe]-Hydrogenase Protein Film Voltammetry at Nanoelectrodes

We have scaled down electrochemical assays of redox-active enzymes enabling us to study small numbers of molecules. Our approach is based on lithographically fabricated Au nanoelectrodes with dimensions down to ca. 70 x 70 nm(2). We first present a detailed characterization of the electrodes using a combination of scanning electron microscopy, cyclic voltammetry, and finite-element modeling. We then demonstrate the viability of the approach by focusing on the highly active [NiFe]-hydrogenase from Allochromatium vinosum immobilized on polymyxin-pretreated Au. Using this system, we successfully demonstrate a distinct catalytic response from less than 50 enzyme molecules. These results strongly suggest the feasibility of using bioelectrochemistry as a new tool for studying redox enzymes at the single-molecule level.

Submicrometer-MOS capacitor with ultra high capacitance biased by Au nanoelectrodes

The ultimate limits of size of the current metal-oxide-semiconductor capacitors can be overcome by preparation of three-dimensional devices that can vertically be biased using one-dimensional metal nanostructures. Here, we present a general and efficient approach to the assembly and integration of Au nanocrystals into functional nanoelectrodes of three-dimensional submicrometer-MOS (0.35 μm2) capacitors, presenting an ultra high capacitance (24±1 pF). The Au nanocrystals were directly produced into a nanoporous template of anodized aluminum oxide that was evaluated, and the electrical characterization of this device corroborates the formation of the MOS capacitor. Flat band voltage is independent of sweep voltage range, and negligible hysteresis of capacitance-voltage curves is observed when sweep voltage ranges from positive to negative and turned again to positive bias. In addition, experimental results match theoretical analysis and indicate the presence of free surface charges stored in the Au nanostructures. The demonstrated ability to control the assembling of the nanocrystals and the results of electrical characterization indicate that the embedded Au nanoelectrodes have a high potential for memory applications based on three-dimensional devices.

Nanoelectrodes for Molecular Devices: A Controllable Fabrication

Nanoelectrodes for Molecular Devices: A Controllable Fabrication

Calculation of the conductance of a finite atomic line of sulfur vacancies created on a molybdenum disulfide surface

Using the elastic-scattering quantum chemistry technique, it is shown that a surface atomic wire fabricated by extracting a line of S surface atoms from the planar $\mathrm{Mo}{\mathrm{S}}_{2}$ lamellar substrate creates enough electronic states in the $\mathrm{Mo}{\mathrm{S}}_{2}$ surface band gap for this wire to have a large conductance. The nature of the surface electronic states introduced by the S vacancies is investigated for increasing numbers of vacancies for a wire length of up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. When contacted by the two Au nanoelectrodes, the wire creates surface pseudoballistic channels and the wire conductance does not decrease with length. The effects of the nanoelectrode-wire distance and of the lateral electrode-wire overlap on the conductance of the wire are also discussed. It is found that the conductance of the junction can be increased threefold by increasing the lateral overlap.

Ultrahigh-sensitive WO3 nanosensor with interdigitated Au nano-electrode for NO2 detection

Ultrahigh-sensitive WO3 nanosensors have been developed by using an interdigitated Au nano-electrode for dilute NO2 detection. The interdigitated Au nano-electrodes with a line spacing of 200 nm and various teeth numbers were fabricated on a SiO2/Si substrate by means of electron beam lithography. A WO3 thin film was deposited on the interdigitated Au nano-electrode to be a WO3 nanosensor. The nanosensor exhibited an extremely high sensor response (Rg/Ra = 28) to 0.01 ppm NO2 with excellent response-recovery characteristics. Interestingly, the sensor response increased with increasing the number of teeth in the interdigitated nano-electrode, i.e., the length of interface between oxide grain and Au electrode. This suggests that the nano-design of electrode structure is important for development of a highly sensitive gas sensor.

Electrochemical nanoelectrodes for advanced investigations of nanostructures

Sophisticated in situ investigation techniques at solid/liquid interfaces on the nanometer length scale using scanning tunnelling microscopy (STM) beyond the simple imaging of surfaces, as for example tunnelling spectroscopy, localized electrodeposition of nanoparticles, or localized electrochemical (catalytic) reaction detection at the STM tip, require in addition to the usual sharpness for high resolution imaging a well-defined geometrical shape and electrochemical behaviour of STM tips. At present, such tips are not available in STM research at solid/liquid interfaces. In this paper, we show how a combined ultra-high-vacuum (UHV) sputtering and field emission microscopy (FEM) technique can be utilized to prepare well-defined nanoelectrode tips. Not only impurities, adsorbates, or surface oxides can be removed by ion-self-sputtering, but the tip geometry is optimized towards an ideal hemispherical shape and the tip apex diameter can be reduced down to a few nanometers, while monitored and controlled in vacuo by the decrease of the corresponding field emission voltage. A detailed investigation is presented for W, Pt, Ni, and Au nanoelectrode tips, which demonstrates the excellent electrochemical as well as imaging properties and the well-defined geometry of such nanoelectrode tips, predestining them for application in any kind of STM based in situ measurement or preparation technique at solid/liquid interfaces.

Dynamically Configurable Biomolecular Nanoarrays

Nanoarrays of distinct DNA and protein biomolecules were fabricated by electrochemically controlling their assembly/release from Au nanoelectrodes on a chip. The surface density, ratio, and activity of the biomolecules assembled on each nanoelectrode in the array can be configured quantitatively and temporally by adjusting the electrochemical potential applied on the nanoelectrode. The dynamically configurable biomolecular nanoarray can potentially activate combinatorial interactions with microbiosystems under the control of an electronic circuit for biological and medical applications.

Fabrication and Characterization of a Nano-Device with V2O5 Nanowires

We fabricated nano-devices with V2O5 nanowires synthesized by using the gel/sol method, and we measured their electrical properties. For a metal-insulator-semiconductor (MIS) structure with V2O5 nanowires on p-type Si substrates, HfO2 dielectrics and Au gate were deposited on the V2O5 nanowires. The electrical properties of this MIS diode were characterized by using capacitance-voltage (C-V) measurements, and the typical C-V hystersis with a flat-band voltage gap of about 3.5 V appeared at room temperature. Also, for an electron tunneling device via nanowires, Au nano-electrodes with a 30 nm gap were fabricated on a SiO2/Si substrate by using electron-beam lithography. This device with V2O5 nanowires inserted into nano-gap electrodes showed an apparent electron tunneling behavior. These electrical properties imply that the use of V2O5 nanowires for memory and tunneling devices may be feasible.

Two-dimensional assembly and local redox-activity of molecular hybrid structures in an electrochemical environment

The self-assembly and redox-properties of two viologen derivatives, N-hexyl-N'-(6-thiohexyl)-4,4'-bipyridinium bromide (HS-6V6-H) and N,N'-bis(6-thiohexyl)-4,4'-bipyridinium bromide (HS-6V6-SH), immobilized on Au(lll)-(1 x 1) macro-electrodes were investigated by cyclic voltammetry, surface enhanced infrared spectroscopy (SEIRAS) and in situ scanning tunneling microscopy (STM). Depending on the assembly conditions one could distinguish three different types of adlayers for both viologens: a low coverage disordered and an ordered "striped" phase of flat oriented molecules as well as a high coverage monolayer composed of tilted viologen moieties. Both molecules, HS-6V6-H and HS-6V6-SH, were successfully immobilized on Au(poly) nano-electrodes, which gave a well-defined redox-response in the lower pA-current range. An in situ STM configuration was employed to explore electron transport properties of single molecule junctions Au(T)/HS-6V6-SH(HS-6V6-H)/Au(S). The observed sigmoidal potential dependence, measured at variable substrate potential E(S) and at constant bias voltage (E(T) - E(S)), was attributed to electronic structure changes of the viologen moiety during the one-electron reduction/re-oxidation process V2+ < -- > V+*. Tunneling experiments in asymmetric, STM-based junctions Au(T)-S-6V6-H/Au(S) revealed current (i(T))-voltage (E(T)) curves with a maximum located at the equilibrium potential of the redox-process V2+ < -- > V+*. The experimental i(T)--E(T) characteristics of the HS-6V6-H-modified tunneling junction were tentatively attributed to a sequential two-step electron transfer mechanism.

Fabrication Technique for Preparing Nanogap Electrodes by Conventional Silicon Processes

A fabrication technique for preparing nanogap electrodes, such as a gold (Au) nano electrode, using conventional silicon (Si) processes–photolithography, etching, thermal oxidation and deposition–is proposed. Stencil substrates are prepared using the Si processes. Then, without requiring complicated technology, nanogap structures can be formed using the technique. Numerous kinds of materials can be selected as an electrode. The mass production of a sensing device for the detection of deoxyribonucleic acid (DNA), or a so-called DNA chip, can be realized at a low cost.

Electrical properties of metal-oxide semiconductor nano-particle device

We have studied the electrical transport properties of two types of devices utilizing metal-oxide semiconductor nano-particles, Cu 2O and Fe 2O 3. The metal-oxide nano-particles are embedded in a polyimide matrix through chemical reaction between the metal thin film and polyamic acid as a precursor of polyimide. To test the electron tunneling via nano-particles, Au nano-electrodes are fabricated on a SiO 2/Si substrate with a 30 nm gap by electron-beam lithography. A single electron tunneling behavior was apparent in the devices with Cu 2O nano-particle inserted into the nano-gap electrodes. Also, a memory effect was measured in a floating-gated memory device structure with Fe 2O 3 nano-particles embedded in a polyimide matrix.

Voltammetry of redox analytes at trace concentrations with nanoelectrode ensembles

Gold nanoelectrodes ensembles (NEEs) have been prepared by electroless plating of Au nanoelectrode elements within the pores of a microporous polycarbonate template membrane. Cyclic voltammograms recorded in (ferrocenylmethyl) trimethylammonium hexafluorophosphate (FA + PF 6 −) solutions showed that these NEEs operate in the “total-overlap” response regime, giving well resolved peak shaped voltammograms. Experimental results show that the faradaic/background currents ratios at the NEE are independent on the total geometric area of the ensemble, so that NEE can be enlarged or miniaturized at pleasure without influencing the very favorable signal/noise ratio. Differential pulse voltammetry (DPV) at the NEE is optimized for direct determinations at trace levels. DPV at NEE allowed the determination (with no preconcentration) of trace amounts of FA +, with a detection limit of 0.02 μM. The use of NEE and DPV in cytochrome c (cyt c) solutions showed the possibility to observe the direct electrochemistry of submicromolar concentration of the protein, even without the need of adding any promoter or mediator.

Direct voltammetry of cytochrome c at trace concentrations with nanoelectrode ensembles

Gold nanoelectrode ensembles (NEEs) are prepared by electroless plating of Au nanoelectrode elements within the pores of a microporous polycarbonate template membrane. The surfaces of these NEEs and also the inner morphology of the gold nanofibers inside the membrane pores are imaged by scanning and transmission electron microscopy. The use of NEEs in micromolar cytochrome c (cyt c) solutions reveals the possibility of observing the direct electrochemistry of the protein, without the need of any promoter or mediator. Cyt c detection limits at NEEs are 1.0 μM by cyclic voltammetry and 0.03 μM by differential pulse voltammetry. The main difference between the voltammetric signals recorded at NEEs in the absence and in the presence of the promoter 4,4 ′-bipyridyl is the more extended dynamic range obtained in the latter case. Quartz crystal microbalance measurements at gold-coated quartz crystals show that, in the absence of promoter, adsorption problems are responsible for the poisoning of the electrode surface. Such adsorption is, however, concentration dependent, so that in diluted solutions (cyt c concentration ⩽20 μM) it becomes negligible. Experimental evidence indicates that the capability of NEEs to detect the direct electrochemistry of cyt c even in the absence of promoters is related to their peculiar property of furnishing well-resolved voltammetric signals in diluted solutions, where this unwanted adsorption is eliminated.

Electrical conduction measurement of thiol modified DNA molecules

We present a novel transport measurement of 60 base pairs of poly(dG)–poly(dC) DNA molecules. Thiol-terminated DNA molecules are chemically anchored at the surface of a Au nanoparticle and this DNA attached Au nanoparticle is self-trapped in between Au nanoelectrodes to make an electrical conduction channel. It provides an automatic electrical conduction channel consisting of electrode–DNA–nanoparticle–DNA–electrode. Due to robust bonding of thiol and Au, this transport channel is stable and reliable. The current–voltage characteristics measured from our device show a nonlinear behavior with voltage gaps comparable to previous experiment using the same molecules.

Construction and Characterization of a Nanowell Electrode Array

Gold nanowell electrode arrays, with a nanowell depth of approximately 600 nm, have been prepared using commercially available anopore membranes as templates. Electrodes were prepared by forming a copper film on anopore, followed by electrodeposition of gold throughout the pore and finally removal of the copper to expose the nanowells. The electrodes were characterized by atomic force microscopy, scanning electron microscopy, and electrochemical methods. Dodecanethiol self-assembled monolayers were found to passivate the electrode. C. R. Martin coined the term “template synthesis” to describe a method in which the pores of a nanoporous membrane act as templates for the synthesis of nanostructures of a desired material. 1 As the pores within these membranes are nearly cylindrical and of uniform diameter, monodisperse nanocylinders of a desired material can be obtained. This method has been applied to the formation of nanowires, 2 metal nanostructures 3 and hollow metal tubules, 4 conductive 5 and insulating polymers, 6 and semiconductors. 7 (For a review of general template-based methods for the preparation of nanomaterials see ref 1.) The formation of gold metal nanostructures by Martin’s group using anopore disks as a template 3 was accomplished by sputtering a thin layer of Ag onto one side of the membrane in order to form an electrode, followed by the galvanostatic deposition of Ag to seal the pores and potentiostatic deposition of Au into the pores on top of the Ag microcylinders. Nitric acid was then used to dissolve away the Ag, leaving Au nanostructures. In this report we discuss a modification of this method which allows us to form an array of Au nanoelectrodes, 200 nm in diameter and which have a controlled depth, embedded within an anopore template. The nanoelectrode array was characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), and electrochemical methods. Passivation of the electrodes with alkanethiol molecules in a self-assembled monolayer (SAM) was also investigated. Anodisc membranes obtained from Whatman were used as the template material for gold nanoelectrode fabrication.