Au Nanoelectrodes Research Articles

Sensitivity enhancement in an in-vitro glucose sensor using gold nanoelectrode ensembles

Enzymatic electrochemical sensor for glucose detection is fabricated based on hydrothermally grown zinc oxide (ZnO) nanorods. The conception of gold (Au) nanoelectrode ensembles (NEEs) is applied to enhance the sensitivity of the electrochemical sensor under investigation. The characterization of as-synthesized ZnO nanorods on Au and indium tin oxide substrates is performed using X-ray diffraction, scanning electron microscopy, and micro-Raman spectroscopy. The current sensitivity of sensors with and without Au NEEs are computed using the cyclic voltammetry and amperometric tests executed for the glucose level ranging from 0.01 to 11.00 mM/L. Current sensitivity of 7.5 µA/mM for a linear range of glucose concentration from 0.01 to 6.5 mM is testified. Response time of 1 s and a low limit of detection of 0.065 mM is reported for the sensor based on Au NEEs. The need of ZnO nanorods in the sensor working electrode, to immobilize enzyme glucose oxidase is justified. The presence of Au NEEs boost the sensor current sensitivity by enhancing the rate of electron transfer during the electrochemical reaction.

Toward More Reliable Measurements of Electron-Transfer Kinetics at Nanoelectrodes: Next Approximation.

Steady-state voltammetry at nanoelectrodes and scanning electrochemical microscopy (SECM) have recently been used to measure kinetics of several rapid heterogeneous electron transfer (ET) reactions. One problem with those experiments was that the dependence of the shape of the steady-state voltammogram on kinetic parameters becomes weak when the reaction rate approaches the diffusion limit. The possibility to fit the same experimental voltammogram using different combinations of the standard rate constant, transfer coefficient, and standard potential results in significant uncertainties in extracted parameter values. In this article, the reliability of the kinetic analysis was improved by obtaining steady-state voltammograms with both oxidized and reduced forms of redox species initially present in solution. Additional improvements were attained by characterizing the nanoelectrode geometry with the atomic force microscope and using water with a very low level of organic contaminants (TOC ≤ 1 ppb). This approach was used to re-evaluate the ET rate constants measured for several electroactive species, including ferrocene, ferrocenemethanol, 7,7,8,8-tetracyanoquinodimethane (TCNQ), and ferrocyanide at Pt electrodes. The obtained standard rate constants are higher than the values measured earlier at Pt and Au nanoelectrodes but comparable to those obtained in recent nanogap/SECM experiments.

Template-assisted growth of transparent plasmonic nanowire electrodes

Self-organized nanowire arrays are confined by glancing-angle Au deposition on nanopatterned glass templates prepared by ion beam sputtering. The semi-transparent 1D nanowire arrays are extended over large cm2 areas and are endowed with excellent electrical conductivity competitive with the best transparent conductive oxides (sheet resistance in the range of 5–20 Ohm sq−1). In addition, the nanowires support localized surface plasmon (LSP) resonances, which are easily tunable into the visible and near infrared spectrum and are selectively excited with incident light polarized perpendicularly to the wires. Such substrates, thus, behave as multifunctional nanoelectrodes, which combine good optoelectronic performance with dichroic plasmonic excitation. The electrical percolation process of the Au nanoelectrodes was monitored in situ during growth at glancing angle, both on flat and nanopatterned glass templates. In the first case, we observed a universal scaling of the differential percolation rate, independently of the glancing deposition angle, while deviations from the universal scaling were observed when Au was confined on nanopatterned templates. In the latter case, the pronounced shadowing effect promotes the growth of locally connected 1D Au nanosticks on the ‘illuminated’ ripple ridges, thus, introducing strong anisotropies with respect to the case of a 2D percolating network.

Mathematical modeling and experimental data of the oxidation of ascorbic acid on electrodes modified by nanoparticles

Physical and mathematical models are proposed which describe the electrochemical transformation of compounds diffusing from the solution to an ensemble of nanoparticles (i.e., nanoelectrodes), located on a solid bulk electrode surface. Electrochemical oxidation of ascorbic acid at solid electrodes (glassy carbon, Graphite Ultra Trace and gold) unmodified and modified with gold nanoparticles (Aunano-ensemble of nanoelectrodes) was experimentally studied. Nanoeffects and diffusion limiting current were observed in the investigation of ascorbic acid oxidation on Aunano electrodes. Nanoeffects manifested themselves in a shift of the oxidation potential towards negative values with decreasing size of the nanoparticles. Agreement between theoretical and experimental results was demonstrated. The shift of the peak potential or half-peak potential can be used to estimate the size of nanoparticles and Gibbs free surface energy and contribution of nanoeffects to the kinetics of the electrode process.

Miniaturized Enzymatic Biosensor via Biofunctionalization of the Insulator of Nanoelectrode Ensembles

AbstractAn innovative second generation enzymatic microbiosensor was developed exploiting the properties of nanoelectrode ensembles (NEEs) prepared by electroless gold deposition in track‐etched polycarbonate (PC) membranes. As a case study, a miniaturized glucose biosensor was developed and characterized. The micro‐NEE glucose biosensor was obtained by immobilizing glucose oxidase (GOx) on the nonconductive PC component of the NEE, while the Au nanoelectrodes were used exclusively as transducers. (Ferrocenylmethyl)trimethylammonium cation (FA+) was used as the redox mediator. The immobilization of GOx on the PC did not affect the electrochemical performances of the NEE nor the catalytic activity of the enzyme. The proposed biosensor showed outstanding analytical performances with a detection limit of 36 µM. The nanostructured biosensor was miniaturized down to a micro‐NEE with overall radius of 400 µm, without any degradation of the analytical performances.

Electrochemical sensor for nitroaromatic type energetic materials using gold nanoparticles/poly(o-phenylenediamine–aniline) film modified glassy carbon electrode

In this work, a novel electrochemical sensor was developed for the detection of nitroaromatic explosive materials, based on a gold nanoparticle-modified glassy carbon (GC) electrode coated with poly(o-phenylenediamine–aniline film) (GC/P(o-PDA-co-ANI)-Aunano electrode). Nitroaromatic compounds were detected through their π-acceptor/donor interactions with o-phenylenediamine–aniline functionalities on the modified electrode surface. The enhanced sensitivities were achieved through π–π and charge-transfer (CT) interactions between the electron-deficient nitroaromatic compounds and σ-/π-donor amine/aniline groups linked to gold nanoparticles (Au-NPs), providing increased binding and preconcentration onto the modified GC-electrodes. Selective determination of nitroaromatic type explosives in the presence of nitramines was enabled by o-PDA and reusability of the electrode achieved by Au-NPs. Calibration curves of current intensity versus concentration were linear in the range of 2.5–40mgL−1 for 2,4,6-trinitrotoluene (TNT) with a detection limit (LOD) of 2.1mgL−1, 2–40mgL−1 for 2,4-dinitrotoluene (DNT) (LOD=1.28mgL−1), 5–100mgL−1 for tetryl (LOD=3.8mgL−1) with the use of the GC/P(o-PDA-co-ANI)-Aunano electrode. For sensor measurements, coefficients of variation of intra- and inter-assay measurements were 0.6% and 1.2%, respectively (N=5), confirming the high reproducibility of the proposed assay. Deconvolution of current contributions of synthetic (TNT+DNT) mixtures at peak potentials of constituents was performed by multiple linear regression analysis to provide high sensitivity for the determination of each constituent. Determination options for all possible mixture combinations of nitroaromatic explosives are presented in this work. The proposed methods were successfully applied to the analysis of nitroaromatics in military explosives, namely comp B, octol, and tetrytol. Method validation was performed against GC–MS on real post-blast residual samples containing both explosives.

Recombinant protein-based nanoscale biomemory devices.

Biomolecular computing devices that are based on the properties of biomolecular activities offer a unique possibility for constructing new computing structures. A new concept of using various biomolecules has been proposed in order to develop a protein-based memory device that is capable of switching physical properties when electrical input signals are applied to perform memory switching. To clarify the proposed concept, redox protein is immobilized on Au nanoelectrodes to catalyze reversible reactions of redox-active molecules, which is controlled electrochemically and reversibly converted between its ON/OFF states. In this review, we summarize recent research towards developing nanoscale biomemory devices including design, synthesis, fabrication, and functionalization based on the proposed concept. At first we analyze the memory function properties of the proposed device at bulk material level and then explain the WORM (write-once-read-many times) nature of the device, later we extend the analysis to multi-bit and multi-level storage functions, and then we focus the developments in nanoscale biomemory devices based on the electron transport of redox molecules to the underlying Au patterned surface. The developed device operates at very low voltages and has good stability and excellent reversibility, proving to be a promising platform for future memory devices.

Electrostatic Spin Crossover in a Molecular Junction of a Single-Molecule MagnetFe2

Spin crossover by means of an electric bias is investigated by spin-polarized density-functional theory calculations combined with the Keldysh nonequilibrium Green's technique in a molecular junction, where an individual single-molecule magnet Fe{2}(acpybutO)(O{2}CMe)(NCS){2} is sandwiched between two infinite Au(100) nanoelectrodes. Our study demonstrates that the spin crossover, based on the Stark effect, is achieved in this molecular junction under an electric bias but not in the isolated molecule under external electric fields. The main reason is that the polarizability of the molecular junction has an opposite sign to that of the isolated molecule, and thus from the Stark effect the condition for the spin crossover in the molecular junction is contrary to that in the isolated molecule.

Electrochemical characterization of nano-sized gold electrodes fabricated by nano-lithography

We report the lithographical fabrication of Au nanoelectrodes, with a geometrical surface area down to 160 nm × 1 μm. The geometrical surface area of the electrodes is verified using electron microscopy and by electrochemistry through the diffusion limited current of reversible redox couples. Moreover, the electrochemically active surface area of the electrodes is determined from the charges transferred in blank voltammetry. We believe these reproducible nanoelectrodes are well suited for use as probes in nano-electrochemistry research.

Robust Au–Ag–Au Bimetallic Atom-Scale Junctions Fabricated by Self-Limited Ag Electrodeposition at Au Nanogaps

Atom-scale junctions (ASJs) exhibit quantum conductance behavior and have potential both for fundamental studies of adsorbate-mediated conductance in mesoscopic conductors and as chemical sensors. Electrochemically fabricated ASJs, in particular, show the stability needed for molecular detection applications. However, achieving physically robust ASJs at high yield is a challenge because it is difficult to control the direction and kinetics of metal deposition. In this work, a novel electrochemical approach is reported, in which Au-Ag-Au bimetallic ASJs are reproducibly fabricated from an initially prepared Au nanogap by sequential overgrowth and self-limited thinning. Applying a potential across specially prepared Au nanoelectrodes in the presence of aqueous Ag(I) leads to preferential galvanic reactions resulting in the deposition of Ag and the formation of an atom-scale junction between the electrodes. An external resistor is added in series with the ASJ to control self-termination, and adjusting solution chemical potential (concentration) is used to mediate self-thinning of junctions. The result is long-lived, mechanically stable ASJs that, unlike previous constructions, are stable in flowing solution, as well as to changes in solution media. These bimetallic ASJs exhibit a number of behaviors characteristic of quantum structures, including long-lived fractional conductance states, that are interpreted to arise from two or more quantized ASJs in series.

Modification of nanoelectrode ensembles by thiols and disulfides to prevent non specific adsorption of proteins

The possibility to functionalize selectively with thiols or disulfides the surface of the gold nanoelectrodes of polycarbonate templated nanoelectrode ensembles (NEEs) is studied. It is shown that the Au nanoelectrodes can be coated by a self assembled monolayer (SAM) of thioctic acid (TA) or 2-mercaptoethanesulfonic (MES) acid. The study of the electrochemical behavior of SAM-modified NEEs by cyclic voltammetry (CV) at different solution pH, using ferrocenecarboxylate as an anionic redox probe (FcCOO −) and (ferrocenylmethyl)trimethylammonium (FA +) as a cationic redox probe, demonstrate that the SAM-modified nanoelectrodes are permselective, in that only cationic or neutral probes can access the SAM-coated nanoelectrode surface. CV, AFM and FTIR-ATR data indicate that proteins such as casein or bovine serum albumin, which are polyanionic at pH 7, adsorb on the surface of NEEs untreated with thiols, tending to block the electron transfer of the ferrocenyl redox probes. On the contrary, the pre-treatment of the NEE with an anionic SAM protects the nanoelectrodes from protein fouling, allowing the detection of well shaped voltammetric patterns for the redox probe. Experimental results indicate that, in the case of MES treated NEEs, the protein is bound only onto the polycarbonate surface which surrounds the nanoelectrodes, while the tips of the gold nanoelectrodes remain protein free.

Transforming the Fabrication and Biofunctionalization of Gold Nanoelectrode Arrays into Versatile Electrochemical Glucose Biosensors

High-density arrays of conducting nanoelectrodes (i.e., nanoelectrode arrays [NEAs]) have been developed on the surface of a single electrode for numerous electrochemical sensing paradigms. However, a scalable fabrication technique and robust biofunctionalization protocol are oftentimes lacking and thus many NEA designs have limited efficacy and overall commercial viability in biosensing applications. In this report, we develop a lithography-free nanofabrication protocol to create large arrays of Au nanoelectrodes on a silicon wafer via a porous anodic alumina template. To demonstrate their effectiveness as electrochemical glucose biosensors, alkanethiol self-assembled monolayers (SAMs) are used to covalently attach the enzyme glucose oxidase to the Au NEA surface for subsequent glucose sensing. The sensitivity and linear sensing range of the biosensor is controlled by introducing higher concentrations of long-chain SAMs (11-mercaptoundecanoic acid: MUA) with short-chain SAMs (3-mercaptopropionic acid: MPA) into the enzyme immobilization scheme. This facile NEA fabrication protocol (that is well-suited for integration into electronic devices) and biosensor performance controllability (via the mixed-length enzyme-conjugated SAMs) transforms the Au NEAs into versatile glucose biosensors. Thus these Au NEAs could potentially be used in important real-word applications such as in health-care and bioenergy where biosensors with very distinct sensing capabilities are needed.

Fabrication of Nanoelectrodes and Metal Clusters by Electrodeposition

Most nanometer-sized electrodes reported to date are made from either Pt or Au. For technical reasons, it is difficult to make nanoelectrodes from many other metals (e.g. Hg) by heat-sealing microwires into glass capillaries or by other established techniques. Such nanoelectrodes can be useful for a wide range of analytical and physicochemical applications from high sensitivity stripping analysis (Hg) to pH nano-sensors to studies of electrocatalysis. In this paper, nanometer-sized metal electrodes are prepared by electrodeposition of Hg or Pt on disk-type, polished or recessed nanoelectrodes. The deposition of Hg is monitored chronoamperometrically to produce near-hemispherical electrodes, which are characterized by voltammetry and scanning electrochemical microscopy (SECM). The well-shaped deposits of a solid metal (Pt) at Au nanoelectrodes are prepared and imaged by scanning electron microscopy (SEM). Catalytic metal clusters can also be prepared using this methodology. Electrodes with the metal surface flush with glass insulator, most suitable for quantitative voltammetric and SECM experiments are fabricated by electrodeposition of a metal inside an etched nanocavity.

Au Disk Nanoelectrode by Electrochemical Deposition in a Nanopore

In this technical note, we report a process in scaling down the fabrication of Au disk nanoelectrodes as small as approximately 4 nm in radii. We have developed a bottom-up approach toward the fabrication of individual disk-shape Au nanoelectrodes. This new approach is based upon electrochemical deposition of Au in a silica nanopore electrode and involves the following four steps. First, a laser-assisted pulling process is employed to fabricate a disk-shape Pt nanoelectrode. Second, a Pt nanopore electrode is obtained by electrochemically etching the Pt from the disk nanoelectrode. Third, a Au metal nanowire is electrochemically deposited using the Pt nanopore electrode as a template. In the last step, the Au electrode is slightly polished to expose a disk-shape Au nanoelectrode, whose size is determined by the size of the initial Pt nanoelectrode. Steady-state voltammetry in the presence of ferrocene has been used to characterize these Au nanoelectrodes. The Au nanoelectrodes are also characterized using cyclic voltammetry in a H2SO4 solution. The results show characteristic peaks corresponding to the formation of Au surface oxides and their subsequent reduction. The Au nanoelectrodes are modified with 6-(ferrocenyl)hexanethiol molecules, and cyclic voltammetry is used to characterize the ferrocene molecules attached at the Au. As an application, we have constructed Au single-nanoparticle electrodes (SNPEs) using the Au disk nanoelectrodes fabricated by electrochemical deposition. Our initial results of such SNPEs show excellent electrochemical response from single Au nanoparticles.

Gold Nanoelectrode Arrays and their Evaluation by Impedance Spectroscopy and Cyclic Voltammetry

A sol-gel strategy is developed to fabricate highly regular Au nanoelectrode arrays (NEAs) consisting of a nanoperforated ultrathin membrane of ZrO(2), which exhibits a well-ordered array of pores (65+/-5) nm in diameter with a mean center-to-center distance of (110+/-10) nm, on a polycrystalline gold surface. The structural properties are investigated by field-emission scanning electron microscopy (FE-SEM), while grazing incidence small-angle X-ray scattering (GISAXS) is used to assess the thickness homogeneity and the period of the array of electrodes. In addition, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are carried out to describe quantitatively the accessibility, electrochemical behavior, and diffusion processes of the gold NEA. A model applying parameters obtained from FE-SEM, CV, and EIS analyses is proposed to simulate the experimental results. A fairly good agreement between the experimental and the simulated data is obtained, thus allowing the deconvolution of the different diffusion regimes at the NEA.

Conductance decay of a surface hydrogen tunneling junction fabricated along a Si(001)-(2×1)-H atomic wire

On a Si(001)-$(2\ifmmode\times\else\texttimes\fi{}1)$-H substrate, electrons tunneling through hydrogen atomic junctions fabricated between two surface dangling-bond (DB) wires are theoretically investigated using the elastic-scattering quantum-chemistry method. The surface states introduced in the Si band gap by removing H atoms from a Si(001)-$(2\ifmmode\times\else\texttimes\fi{}1)$-H surface were calculated and also analyzed using a simple tight-binding model. The two-channel surface conductance of a DB wire results from a combination of through-space and through-lattice electronic couplings between DB states. The conductance of the DB wire-H-junction-DB wire structure decreases exponentially with the length of H junction with an inverse decay rate ranging from 0.20 to $0.23\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$, depending on the energy. When the DB wire-H-junction-DB wire structure is contacted by Au nanoelectrodes, the transmission resonances corresponding to the DB wire states split, demonstrating a coupling of the DB wires through short surface hydrogen atomic junctions. This splitting decreases with the length of H junction between the DB wires with an inverse decay length ranging from 0.22 to $0.44\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$, indicating that such an atomic scale surface tunneling junction is not a very good insulator.

Fractional quantum conductance values in Au nanoelectrodes due to hydrogen adsorption

Hydrogen adsorption in gold nanocontact electrodes in electrochemical solution is experimentally discerned. This is performed with gold nanocontact conductance histograms in an electrochemical environment in which both the electrochemical potential and the electrolyte type are varied. Different salts, acids, and hydrogen peroxide electrolytes are studied. Salts and acids exhibit at negative electrochemical potentials different fractional quantum conductance histograms peaks associated to extra stable structures due to H adsorption while these peaks do not appear for H 2O 2 where electron transfer between solution and electrodes occurs without hydrogen formation or hydrogen adsorption on the gold electrode.

A new hydrogen peroxide biosensor based on gold nanoelectrode ensembles/multiwalled carbon nanotubes/chitosan film-modified electrode

Gold nanoelectrode ensembles were produced by electrodeposition using multiwalled carbon nanotubes (MWNTs) as template. A new third generation amperometric biosensor for hydrogen peroxide was developed based on adsorption of horseradish peroxidase (HRP) at the glassy carbon (GC) electrode modified with Au nanoelectrode ensembles/multiwalled carbon nanotubes/chitosan film. The resulting HRP biosensor offered an excellent detection for hydrogen peroxide at −0.11 V with a linear response range of 2.08 × 10 −7 to 7.6 × 10 −3 M with a correlation coefficient of 0.998, and response time <5 s. The detection limit was 1.02 × 10 −7 M at 3 σ. The biosensor displays rapid response, expanded linear response range, and excellent repeatability. The simple and fast fabrication of the sensor makes it superior to other techniques.

Scanning Electrochemical Microscopy with Gold Nanotips: The Effect of Electrode Material on Electron Transfer Rates

The disk-type, polished Au nanoelectrodes were prepared, characterized, and used as tips in the scanning electrochemical microscope (SECM). Kinetic parameters were measured for several rapid heterogeneous electron transfer (ET) reactions and compared to those obtained previously at Pt nanoelectrodes. The kinetics of most investigated ET processes were found to be independent of the electrode material. In contrast to the previously reported results, the rate constant of hexaammineruthenium(III) reduction was somewhat higher at Pt than at Au electrodes in different electrolyte solutions. This finding suggests that this reaction exhibits some degree of nonadiabaticity.

Polymer Membrane Stabilized Gold Nanostructures Modified Electrode and Its Application in Nitric Oxide Detection

Nafion polymer/gold nanostructures film that can be utilized as a proficient electrochemical sensor for nitric oxide was prepared by an electrochemical process, forming the gold nanostructures (Aunano) through infiltration into a Nafion (Nf) matrix preassembled on an electrode. The formation of gold nanostructures was monitored by the in situ spectroelectrochemical method. The in situ absorption spectra of Aunano showed systematic and uniform formation of gold nanostructures at the Nafion (Nf−Aunano)-modified electrode. The electrochemically formed Nf−Aunano was characterized by UV−visible spectroscopy, X-ray diffraction, scanning electron microscope, transmission electron microscope, and electrochemical techniques. The surface plasmon absorption spectra recorded for the wet and dry Nf−Aunano composite film showed the interaction between the gold nanostructures and the swelled polymer matrix. The longitudinal surface plasmon band and the TEM images observed for the Nf−Aunano showed the formation of nanorod-like and Y-shaped gold nanostructures in the Nafion matrix. In addition to the nanoparticles, the edge-to-edge interactions lead to the formation of 1D assembly. The electrical communication between the gold nanostructures embedded in the Nafion film improved the electrocatalytic properties of the modified electrode toward NO detection. The Nf−Aunano electrode showed excellent sensitivity for NO detection with the experimental detection limit of 1 nM. The present Nf−Aunano electrode is very simple to fabricate and is stable, sensitive, and reproducible.