Gold Nanoelectrodes Research Articles

Transition from Strong to Weak Electronic Coupling in a Single-Molecule Junction.

We have investigated charge transport in single-molecule junctions using gold nanoelectrodes at room and cryogenic (10K) temperatures. A statistical analysis of the low-bias conductance, measured during the stretching of the molecular junctions, shows that the most probable single-molecule conductance is insensitive to the temperature as expected for off-resonant coherent transport. Low-temperature current-voltage measurements show that these junction conformations have a smooth tunnelinglike shape. While separating the electrodes further we find that, in about one-fourth of the cases, the junction switches in an abrupt way to a configuration with I-V characteristics exhibiting a gap around zero bias and resonances at finite bias. The analysis of the I-V shape and of the conductance distance dependence suggests a stretching-induced transition from the strong to the weak electronic coupling regime. The transition involves a large renormalization of the injection barrier and of the electronic coupling between the molecule and the electrodes.

Array of recessed gold nanoelectrodes formed with polymethylmethacrylate for individual detection of ascorbic acid, dopamine and uric acid

A series of recessed gold nanoelectrode arrays with different array densities are patterned on polymethylmethacrylate (PMMA) coated gold substrate by precisely controlled patterning through electron beam lithography. The influence of the nanoelectrode density of the arrays and scan rate on the diffusion regimes and hence electrochemical behaviors are systematically investigated. It is demonstrated that the electrostatic interaction exists between the oxygen-containing groups of the PMMA nanocylinder and the charged analytes in solution. The difference in the electrostatic interaction of dopamine (DA), ascorbic acid (AA), and uric acid (UA) with PMMA nanocylinders leads to differentiation of the oxidation of the three species. Thus, an electrochemical sensing platform for the detection of individual components in a mixture of DA, AA, and UA is developed. The linear ranges and detection limits of individual components in the mixture are 3.5–125μM and 0.66μM for DA, 30–190μM and 7.5μM for AA, 20–170μM and 6.38μM for UA, respectively, on the array electrode.

Gold nanoparticles in aqueous solutions: influence of size and pH on hydrogen dissociative adsorption and Au(iii) ion reduction.

The shift of the localized surface plasmon resonance (LSPR) band of gold nanoparticles to shorter wavelengths upon saturation of the hydrosol with hydrogen is used as a tool to study the electrochemical processes on the particle surface. It is shown that dissociative adsorption of hydrogen takes place on the surface of a particle and results in the migration of a proton into the dispersion medium, while the electron remains on the nanoparticle, i.e., a hydrogen-like nanoelectrode is formed. It is shown that Au(iii) ions can be reduced on the gold nanoelectrodes. A thermodynamic scheme explaining the shift of the LSPR band is used to explain the peculiarities of the Au(iii) ion reduction. The reduction rate does not depend on the ion concentration and varies linearly with pH. The observed correlations are explained in terms of a simple model of electrochemical processes taking place on the nanoparticle as an electrode. It is shown that with an increase in the particle size, its capacity for dissociative adsorption of hydrogen decreases and the Au(iii) reduction slows down.

Probing the local environment of a single OPE3 molecule using inelastic tunneling electron spectroscopy.

We study single-molecule oligo(phenylene ethynylene)dithiol junctions by means of inelastic electron tunneling spectroscopy (IETS). The molecule is contacted with gold nano-electrodes formed with the mechanically controllable break junction technique. We record the IETS spectrum of the molecule from direct current measurements, both as a function of time and electrode separation. We find that for fixed electrode separation the molecule switches between various configurations, which are characterized by different IETS spectra. Similar variations in the IETS signal are observed during atomic rearrangements upon stretching of the molecular junction. Using quantum chemistry calculations, we identity some of the vibrational modes which constitute a chemical fingerprint of the molecule. In addition, changes can be attributed to rearrangements of the local molecular environment, in particular at the molecule–electrode interface. This study shows the importance of taking into account the interaction with the electrodes when describing inelastic contributions to transport through single-molecule junctions.

A Sensitive Electrochemiluminescence Immunosensor for Celiac Disease Diagnosis Based on Nanoelectrode Ensembles.

We report here the design of a novel immunosensor and its application for celiac disease diagnosis, based on an electrogenerated chemiluminescence (ECL) readout, using membrane-templated gold nanoelectrode ensembles (NEEs) as a detection platform. An original sensing strategy is presented by segregating spatially the initial electrochemical reaction and the location of the immobilized biomolecules where ECL is finally emitted. The recognition scaffold is the following: tissue transglutaminase (tTG) is immobilized as a capturing agent on the polycarbonate (PC) surface of the track-etched templating membrane. It captures the target tissue transglutaminase antibody (anti-tTG), and finally allows the immobilization of a streptavidin-modified ruthenium-based ECL label via reaction with a suitable biotinylated secondary antibody. The application of an oxidizing potential in a tri-n-propylamine (TPrA) solution generates an intense and sharp ECL signal, suitable for analytical purposes. Voltammetric and ECL analyses evidenced that the ruthenium complex is not oxidized directly at the surface of the nanoelectrodes; instead ECL is generated following the TPrA oxidation, which produces the TPrA•+ and TPrA• radicals. With NEEs operating under total overlap diffusion conditions, high local fluxes of these reactive radicals are produced by the nanoelectrodes in the immediate vicinity of the ECL labels, so that they efficiently generate the ECL signal. The radicals can diffuse over short distances and react with the Ru(bpy)32+ label. In addition, the ECL emission is obtained by applying a potential of 0.88 V versus Ag/AgCl, which is about 0.3 V lower than when ECL is initiated by the electrochemical oxidation of Ru(bpy)3(2+). The immunosensor provides ECL signals which scale with anti-tTG concentration with a linearity range between 1.5 ng·mL–1 and 10 μg·mL–1 and a detection limit of 0.5 ng·mL–1. The sensor is finally applied to the analysis of anti-tTG in human serum samples, showing to be suitable to discriminate between healthy and celiac patients.

Nano-fabrication of molecular electronic junctions by targeted modification of metal-molecule bonds.

Reproducibility, stability and the coupling between electrical and molecular properties are central challenges in the field of molecular electronics. The field not only needs devices that fulfill these criteria but they also need to be up-scalable to application size. In this work, few-molecule based electronics devices with reproducible electrical characteristics are demonstrated. Our previously reported 5 nm gold nanoparticles (AuNP) coated with ω-triphenylmethyl (trityl) protected 1,8-octanedithiol molecules are trapped in between sub-20 nm gap spacing gold nanoelectrodes forming AuNP-molecule network. When the trityl groups are removed, reproducible devices and stable Au-thiol junctions are established on both ends of the alkane segment. The resistance of more than 50 devices is reduced by orders of magnitude as well as a reduction of the spread in the resistance histogram is observed. By density functional theory calculations the orders of magnitude decrease in resistance can be explained and supported by TEM observations thus indicating that the resistance changes and strongly improved resistance spread are related to the establishment of reproducible and stable metal-molecule bonds. The same experimental sequence is carried out using 1,6-hexanedithiol functionalized AuNPs. The average resistances as a function of molecular length, demonstrated herein, are comparable to the one found in single molecule devices.

New functional materials based on conductive polymer—metal complexes modified with metallic nanoelectrodes

The results of the synthesis and investigation of new composite functional materials are presented. The new materials are conductive polymer layers of the cobalt, nickel, and palladium complexes with the Schiff bases (poly[MSchiff)]) with gold and platinum nanoelectrodes inserted into the layers using the electrochemical deposition method. The materials were studied by cyclic voltammetry, impedance spectroscopy, scanning electron microscopy with X-ray microanalysis, and a quartz crystal microbalance. The composite materials poly[Ni(Schiff)]/Pt and poly[Co(Schiff)]/Au exhibit catalytic activity in the reactions of hydrogen evolution and electroreduction of molecular oxygen, respectively.

Single-Molecule Break Junctions Based on a Perylene-Diimide Cyano-Functionalized (PDI8-CN2) Derivative.

In this letter, we report the single-molecule conductance properties of a cyano-functionalized perylene-diimide derivative (PDI8-CN2) investigated with gold nano-electrodes. This molecule is of large interest for the fabrication of high-performance and air-stable n-type organic field-effect transistors. Low-bias experiments performed on mechanically controllable break junctions reveal the presence of two different values of the single-molecule conductance, which differ by about two orders of magnitudes. Up to date, this feature was never observed for other perylene-diimide compounds having alternative chemical moieties attached to the basic aromatic core. Theoretical calculations suggest that the highest single-molecule conductance value here observed, comprised between 10−2 and 10−3 G0, is related to a charge transport path directly linking the two cyano groups.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-1011-3) contains supplementary material, which is available to authorized users.

Electrical properties and mechanical stability of anchoring groups for single-molecule electronics.

We report on an experimental investigation of transport through single molecules, trapped between two gold nano-electrodes fabricated with the mechanically controlled break junction (MCBJ) technique. The four molecules studied share the same core structure, namely oligo(phenylene ethynylene) (OPE3), while having different aurophilic anchoring groups: thiol (SAc), methyl sulfide (SMe), pyridyl (Py) and amine (NH2). The focus of this paper is on the combined characterization of the electrical and mechanical properties determined by the anchoring groups. From conductance histograms we find that thiol anchored molecules provide the highest conductance; a single-level model fit to current–voltage characteristics suggests that SAc groups exhibit a higher electronic coupling to the electrodes, together with better level alignment than the other three groups. An analysis of the mechanical stability, recording the lifetime in a self-breaking method, shows that Py and SAc yield the most stable junctions while SMe form short-lived junctions. Density functional theory combined with non-equlibrium Green’s function calculations help in elucidating the experimental findings.

Light-Induced Switching of Tunable Single-Molecule Junctions.

A major goal of molecular electronics is the development and implementation of devices such as single‐molecular switches. Here, measurements are presented that show the controlled in situ switching of diarylethene molecules from their nonconductive to conductive state in contact to gold nanoelectrodes via controlled light irradiation. Both the conductance and the quantum yield for switching of these molecules are within a range making the molecules suitable for actual devices. The conductance of the molecular junctions in the opened and closed states is characterized and the molecular level E 0, which dominates the current transport in the closed state, and its level broadening Γ are identified. The obtained results show a clear light‐induced ring forming isomerization of the single‐molecule junctions. Electron withdrawing side‐groups lead to a reduction of conductance, but do not influence the efficiency of the switching mechanism. Quantum chemical calculations of the light‐induced switching processes correlate these observations with the fundamentally different low‐lying electronic states of the opened and closed forms and their comparably small modification by electron‐withdrawing substituents. This full characterization of a molecular switch operated in a molecular junction is an important step toward the development of real molecular electronics devices.

Modeling contact formation between atomic-sized gold tips via molecular dynamics

The formation and rupture of atomic-sized contacts is modelled by means of molecular dynamics simulations. Such nano-contacts are realized in scanning tunnelling microscope and mechanically controlled break junction experiments. These instruments routinely measure the conductance across the nano-sized electrodes as they are brought into contact and separated, permitting conductance traces to be recorded that are plots of conductance versus the distance between the electrodes. One interesting feature of the conductance traces is that for some metals and geometric configurations a jump in the value of the conductance is observed right before contact between the electrodes, a phenomenon known as jump-to-contact. This paper considers, from a computational point of view, the dynamics of contact between two gold nano-electrodes. Repeated indentation of the two surfaces on each other is performed in two crystallographic orientations of face-centred cubic gold, namely (001) and (111). Ultimately, the intention is to identify the structures at the atomic level at the moment of first contact between the surfaces, since the value of the conductance is related to the minimum cross-section in the contact region. Conductance values obtained in this way are determined using first principles electronic transport calculations, with atomic configurations taken from the molecular dynamics simulations serving as input structures.

Dielectrophoretic trapping of multilayer DNA origami nanostructures and DNA origami‐induced local destruction of silicon dioxide

Electrophoresis 2015, 36, 255–262. DOI: 10.1002/elps.201400323 DNA origami is a widely used method for fabrication of complex and custom-shaped nanostructures. To further utilize these versatile nanoarchitectures, we demonstrate guiding and anchoring of individual DNA origamis on a chip. Different types of 3D multilayer DNA origami structures were trapped and immobilized on gold nanoelectrodes by AC-dielectrophoresis (DEP). Trapping of single brick-like objects is shown in the back-image as well as the implemented electrode geometry. We observed that structural deformations of DNA origamis during the DEP trapping procedure are highly dependent on the shape of the object and the fundamental construction of the origami design. In addition, the electrical conductivity of single unmodified brick-like origami structures immobilized between the nanoelectrodes revealed to be negligible. However, we found out that trapping of the same design equipped with densely-packed thiol-linkers (see up-left inset), tended to induce an etched “nanocanyon” in the SiO2 substrate precisely at the location of the trapped origami (see low right inset). The results demonstrate that the DEP-trapping technique can be exploited in assembling of complex origami geometries. Moreover, DNA origamis could be used in deformation of the substrates onto which they are attached.

Ultrasensitive electrochemical biomolecular detection using nanostructured microelectrodes.

Electrochemical sensors have the potential to achieve sensitive, specific, and low-cost detection of biomolecules--a capability that is ever more relevant to the diagnosis and monitored treatment of disease. The development of devices for clinical diagnostics based on electrochemical detection could provide a powerful solution for the routine use of biomarkers in patient treatment and monitoring and may overcome the many issues created by current methods, including the long sample-to-answer times, high cost, and limited prospects for lab-free use of traditional polymerase chain reaction, microarrays, and gene-sequencing technologies. In this Account, we summarize the advances in electrochemical biomolecular detection, focusing on a new and integrated platform that exploits the bottom-up fabrication of multiplexed electrochemical sensors composed of electrodeposited noble metals. We trace the evolution of these sensors from gold nanoelectrode ensembles to nanostructured microelectrodes (NMEs) and discuss the effects of surface morphology and size on assay performance. The development of a novel electrocatalytic assay based on Ru(3+) adsorption and Fe(3+) amplification at the electrode surface as a means to enable ultrasensitive analyte detection is discussed. Electrochemical measurements of changes in hybridization events at the electrode surface are performed using a simple potentiostat, which enables integration into a portable, cost-effective device. We summarize the strategies for proximal sample processing and detection in addition to those that enable high degrees of sensor multiplexing capable of measuring 100 different analytes on a single chip. By evaluating the cost and performance of various sensor substrates, we explore the development of practical lab-on-a-chip prototype devices. By functionalizing the NMEs with capture probes specific to nucleic acid, small molecule, and protein targets, we can successfully detect a wide variety of analytes at clinically relevant concentrations and speeds. Using this platform, we have achieved attomolar detection levels of nucleic acids with overall assay times as short as 2 min. We also describe the adaptation of the sensing platform to allow for the measurement of uncharged analytes--a challenge for reporter systems that rely on the charge of an analyte. Furthermore, the capabilities of this system have been applied to address the many current and important clinical challenges involving the detection of pathogenic species, including both bacterial and viral infections and cancer biomarkers. This novel electrochemical platform, which achieves large molecular-to-electrical amplification by means of its unique redox-cycling readout strategy combined with rapid and efficient analyte capture that is aided by nanostructured microelectrodes, achieves excellent specificity and sensitivity in clinical samples in which analytes are present at low concentrations in complex matrices.

Special Section on Spectroscopy, Scattering, and Imaging Techniques for Nanostructured Materials

Special Section on Spectroscopy, Scattering, and Imaging Techniques for Nanostructured Materials

Elastic Modulus Measurements on Large Diameter Nanowires Using a Nano-Assembled Platform

This paper presents atomic force spectroscopy (AFM) results from large diameter nanowires (NWs), which range in radius from 150 nm to 300 nm, within a nano-assembled platform. The nanomechanical platform is constructed by assembling single NWs across pairs of gold nano-electrodes using dielectrophoresis and contains a short, suspended segment of the NW (in air) between the assembly electrodes. Atomic force microscope (AFM) force spectroscopy measurements are obtained by indenting the NW within this suspended segment and result in deformation of the NW involving a combination of both, bending and nano-indentation modes. This paper demonstrates the measurement technique using lithium iron phosphate NWs as a model system and presents a finite element model to extract the Young's modulus from nanomechanical data. The estimated Young's modulus of this material, which is an electrode material system of interest for next-generation lithium-ion batteries, was found to be diameter dependent and was observed to range in values between 100 MPa and 575 MPa.

Fluorescein-labeled “arch-like” DNA probes for electrochemical detection of DNA on gold nanoparticle-modified gold electrodes

In the present study, a gold nanoparticle-modified gold electrode (nanogold electrode) was used to develop a novel fluorescein electrochemical DNA biosensor based on a target-induced conformational change. The nanogold electrode was obtained by electrodepositing gold nanoparticles onto a bare gold electrode. This modification not only immobilized probe oligonucleotides, but also adsorbed fluorescein onto the surface of the gold nanoparticles to form an “arch-like” structure. This article compares the electrochemical signal changes caused by the hybridization of “arch-like” DNA on nanogold electrode and linear DNA on bare gold electrode. The results showed that the adsorption effect of nanogold can enhance the sensitivity of the sensor. The linear range of target ssDNA is from 2.0×10−9M to 2.0×10−8M with a correlation coefficient of 0.9956 and detection limit (3σ) of 7.10×10−10M. Additionally, the specificity and hybridization response of this simple sensor were investigated.

Rapid and highly sensitive detection of Enterovirus 71 by using nanogold-enhanced electrochemical impedance spectroscopy

Enterovirus 71 (EV71) infection is an emerging infectious disease causing neurological complications and/or death within two to three days after the development of fever and rash. A low viral titre in clinical specimens makes the detection of EV71 difficult. Conventional approaches for detecting EV71 are time consuming, poorly sensitive, or complicated, and cannot be used effectively for clinical diagnosis. Furthermore, EV71 and Coxsackie virus A16 (CA16) may cross react in conventional assays. Therefore, a rapid, highly sensitive, specific, and user-friendly test is needed. We developed an EV71-specific nanogold-modified working electrode for electrochemical impedance spectroscopy in the detection of EV71. Our results show that EV71 can be distinguished from CA16, Herpes simplex virus, and lysozyme, with the modified nanogold electrode being able to detect EV71 in concentrations as low as 1 copy number/50 μl reaction volume, and the duration between sample preparation and detection being 11 min. This detection platform may have the potential for use in point-of-care diagnostics.

Highly sensitive 3D gold nanotube ensembles: Application to electrochemical determination of metronidazole

Three-dimensional gold nanoelectrode ensembles (3D GNE) have proven to be promising nanoelectrodes by representing much higher sensitivity compared to both their 2D nanostructures and bulk counterparts. The sensitivity of 3D gold nanotubes (GNTs) fabricated through electrodeposition inside the pores of polycarbonate templates, was examined toward metronidazole (MTZ) as one of their pharmaceutical applications. The electrochemical behavior of MTZ at the 3D GNT-modified electrode was discussed in detail through cyclic voltammetry (CV) which suggested an irreversible reduction of nitro group to the corresponding hydroxylamine and a subsequently reversible redox peak for the corresponding product. This latter redox peak was selected for MTZ determination which has not been reported, so far. Square wave voltammetry (SWV) was used as a powerful tool for detection of MTZ in different concentrations. Under the optimal conditions, the highly sensitive 3D GNT showed a wide linear range from 1nM to 2μM with a sufficiently low detection limit of 0.1nM demonstrating promising results for the future usage in clinical applications.

Gas-assisted focused ion beam fabrication of gold nanoelectrode arrays in electron-beam evaporated alumina films for microfluidic electrochemical sensors

This work reports the fabrication details used to prepare gold nanoelectrode arrays as working electrodes in a microfluidic electrochemical sensor system. We emphasize on the design criteria and the methods used to construct the nanoelectrode arrays for the purpose of enhancing mass transport of the redox species to the electrodes. The fabrication techniques include electron-beam evaporation of Ti and Au to create the electrode platform on a glass substrate, followed by electron-beam evaporation of thick alumina films as a passivation layer. I2-assisted focused ion beam (FIB) milling is employed to produce high-aspect-ratio pores in the alumina films. The final nanopores with diameters around 60–120nm are achieved by ion beam sculpting after the initial milling process. The nanopores are filled with Au via electrodeposition to obtain a nanoelectrode structure. The current responses of Ru(NH3)63+/Ru(NH3)62+ redox species measured at three-electrode cells in polydimethylsiloxane (PDMS) microfluidic chips are investigated by cyclic voltammetry (CV). The current density obtained at the nanoelectrode arrays is increased by two orders of magnitude compared to that obtained from the microelectrode system. In addition, due to the effect of enhanced mass transport, the voltammetric response curve acquired from the nanoelectrode array exhibits large deviations from the classical diffusion-limited current transport theory.

Electrochemical determination of Verapamil with a microchip embedded with gold nanoelectrode ensemble electrodes

Verapamil is commonly used for the treatments of supraventricular arrhythmias, angina and hypertension. The chemical structure and properties of Verapamil are stable and show no significant electrochemical activity without treating Verapamil with strong chemicals or high-energy radio-waves. This research develops a simple and effective method to oxidize Verapamil into electrochemical active derivatives with a novel ozone pre-treatment process. The oxidation reaction of Verapamil is investigated and confirmed with the UV–vis spectrometry and mass-spectrometry analyses. In order to further quantitatively evaluate the concentration of Verapamil, the oxidized Verapamil products are then detected using an electrochemical microchip with a gold nanoelectrode ensemble electrode (GNEE) as the working electrode. Enhanced by the excellent electrochemical catalytic properties of the GNEE working electrode, a significant redox peak is observed in the cyclic voltammetry test for the ozone treated Verapamil. Compared to the conventional analytical method of the UV absorption approach, the method developed greatly improves the sensitivity for detecting Verapamil without using complex pre-treatment process. The limit of detection for analyzing Verapamil with the method developed is 1.0ng/mL, which is 125-fold lower than the reported serum concentration of 125.0ng/mL. The experimental measured linear dynamic range is from 1.0ng/mL to 100.0μg/mL with a correlation factor of 0.99. The development method provides a rapid and efficient way to quantitatively analyze Verapamil for clinical applications.