DNA hybridization "turns on" electro-catalysis at gold electrodes

The efficiency of electro-catalysis occurring at DNA-modified gold electrodes is highly dependently on the density of DNA monolayers, as a result, DNA hybridization can "turn on" electro-catalysis by increasing the DNA surface density.     

Unmediated by DNA electron transfer in redox-labeled DNA duplexes end-tethered to gold electrodes

Electron transfer (ET) between gold electrodes and redox-labeled DNA duplexes, immobilized onto the electrodes through the alkanethiol linker at the 3'-end and having internal either methylene blue (MB) or anthraquinone (AQ) redox labels, was shown to depend on the redox label charge and the way the redox label is linked to DNA. For loosely packed DNA monolayers, the conjugation of the positively charged MB to DNA through the long and flexible alkane linker provided ET whose kinetics was formally governed by the diffusion of the redox label to the negatively charged electrode surface. For the uncharged AQ label no ET signal was detected. The conjugation of AQ to DNA through the short and more conductive acetylene linker did not provide the anticipated DNA-mediated ET to the AQ-moiety: ET appeared to be low-efficient if any in the studied system, for which no intercalation of AQ within the DNA duplex occurred. The ET communication between the electrode and AQ, built in DNA through the acetylene linker, was achieved only when Ru(NH(3))(6)(3+) molecules were electrostatically attached to the DNA duplex, thus forming the electronic wire. These results are of particular importance both for the fundamental understanding of the interfacial behavior of the redox labeled DNA on electrodes and for the design of biosensors exploiting a variation of ET properties of DNA in the course of hybridization.     

Self-assembled monolayers and electrostatically self-assembled multilayers of polyalkylviologens on sulfonate-modified gold and indium–tin–oxide electrodes

Self-assembly (SA) of polyviologens with linear alkyl spacers PVn (n = 3–10) on sulfonate-primed gold and ITO electrode surfaces has been investigated by CV, QCM, XPS and AFM. The polymers form stable and relatively dense monolayers even from 10⁻⁴ M PVn concentration. The viologen surface density is decreased as n is increased but for n = 10 and on alkylsulfonate-primed surfaces a strong adsorption takes place, with decreased redox potential and very narrow cyclic voltammogram of the absorbed layer due to organized structures on the surface.Stable and regular electrostatically self-assembled (ESA) multilayers are built on sulfonate-primed surfaces with PV3 and polysulfonates. From XPS analysis, the composition of the PVn/polysulfonate multilayers corresponds to a 1:1 ratio of polyanion and polycation charges, i.e., no extra non-polymeric ion is present, independently from the alkane chain length. The rate of electron transfer within the multilayers (diffusion coefficient = ca. 10⁻⁹ cm² s⁻¹) is higher than for analogous bulk materials.     

Detection of DNA immobilized on bare gold electrodes and gold nanoparticle-modified electrodes via electrogenerated chemiluminescence using a ruthenium complex as a tag

Electrogenerated chemiluminescence (ECL) for DNA hybridization detection is demonstrated based on DNA that was self-assembled onto a bare gold electrode and onto a gold nanoparticles modified gold electrode. A ruthenium complex served as an ECL tag. Gold nanoparticles were self-assembled on a gold electrode associated with a 1,6-hexanedithiol monolayer. The surface density of single stranded DNA (ssDNA) on the gold nanoparticle modified gold electrode was 4.8?×?10¹⁴ molecules per square centimeter which was 12-fold higher than that on the bare gold electrode. Hybridization was induced by exposure of the target ssDNA gold electrode to the solution of ECL probe consisting of complementary ssDNA tagged with ruthenium complex. The detection limit of target ssDNA on a gold nanoparticle modified gold electrode (6.7?×?10^?12 mol L^?1) is much lower than that on a bare gold electrode (1.2?×?10^?10 mol L^?1). The method has been applied to the detection of the DNA sequence related to cystic fibrosis. This work demonstrates that employment of gold nanoparticles self-assembled on a gold electrode is a promising strategy for the enhancement of the sensitivity of ECL detection of DNA.     

Electroenzymatic reactions with sorbitol dehydrogenase on gold electrodes

Sorbitol dehydrogenase (SDH) originating from recombinant Escherichia coli cells is immobilized on gold electrodes. First of all, (4-carboxy-2,5,7-trinitrofluorenyliden)malon-nitrile (CTFM) is adsorbed on the surface as mediator. In a second step, the cofactor β-nicotinamide adenine dinucleotide (NAD⁺) is immobilized on the gold electrode. Due to the formation of a complex between the mediator and the cofactor, the electron transfer rate can be enhanced by adding calcium ions to the buffer. The immobilization of NAD⁺ and SDH on the surface has been achieved by cross-linking with the glutaraldehyde/bovine serum albumin system. The successful biofunctionalization is monitored by cyclic voltammetry.Paper presented at the “Jahrestagung der Fachgruppe Angewandte Electrochemie der Gesellschaft Deutscher Chemiker, Düsseldorf, 11.-14.09.2005”.     

Formation of cyanuric acid from cyanate adsorbed at gold electrodes

We report the formation of cyanuric acid species at Au(111) electrodes in cyanate-containing solutions, due to the electroless trimerization of isocyanic acid. Similar experimental bands in the range between 1300 and 1900 cm^− 1 are observed in Surface Enhanced Infrared Reflection Spectroscopy experiments under Attenuated Total Reflection conditions (ATR-SEIRAS) with solutions containing either cyanate anions at high concentration or cyanuric acid. The experimental frequencies agree well with those obtained from Density Functional Theory (DFT) calculations for the adsorbed cyanurate anion, bonded to the metal in a tridentate configuration, with its molecular plane perpendicular to the metal surface.     

Size-controlled fabrication of nanometer-sized gold electrodes with polystyrene coating

We present a simple, convenient procedure for the fabrication of nanometer-sized gold electrodes with the ability to control the electrode size at the construction stage. The electrodes are prepared by etching a gold wire, coating it with a polystyrene film, and then removing the film from the tip apex by thermal stripping in an aqueous solution in conjunction with in situ monitoring of the exposed electrode area by cyclic voltammetry measurements. It is demonstrated that the method produces point-like electrodes with precise control of the apparent electrode radius within a few nanometers.     

Surface modification of gold electrodes with anthraquinone diazonium cations

Electrochemical grafting of anthraquinone (AQ) groups to gold electrodes was carried out in acetonitrile (ACN) and in aqueous acidic media (0.05 M H₂SO₄). For the first time, the covalent attachment of AQ to gold is demonstrated. Electrochemical quartz crystal microbalance (EQCM), atomic force microscopy (AFM) and cyclic voltammetry (CV) were used to characterise the AQ-modified Au electrodes. Electrografting from both solutions containing the corresponding diazonium salt yielded a strongly attached AQ layer. AFM examination showed that a multilayer AQ film was formed. The CV behaviour of the modified electrodes was tested in 0.1 M KOH and a quasi-reversible response was observed.     

Electrochemical and microgravimetric studies of poly[3,4-ethylenedioxythiophene]-tyrosinase biocomposite material electrodeposited onto gold electrodes by a sinusoidal voltages method

The electrodeposition of poly(3,4-ethylenedioxythiophene)-tyrosinase (PEDOT-Ty) biocomposite material onto gold electrode has been achieved by means of a sinusoidal voltages (SV) method. The SV method consists in the superimposition of a sinusoidal voltage (sin wave) with fixed frequency and amplitude onto a d.c. potential. The influence of electrochemical parameters like frequency and amplitude of SV signal, d.c. potential value, deposition time, on the electrodeposition process has been investigated. The biocomposite material has been prepared as a thin layer onto quartz crystals coated with gold. The frequency change of the quartz crystal during the electrodeposition of the biocomposite material was recorded simultaneously with the measured current response. The morphology of the deposited PEDOT-Ty coatings was investigated by scanning electron microscopy. The electrochemical behavior of the biocomposite material in aqueous solution was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The use of d.c. potential value of 0.6 V revealed the contribution of the SV component to the electrodeposition process. The PEDOT-Ty modified electrode was used as an electrochemical biosensor in the voltammetric determination of dopamine in the presence of hydroquinone. The analytical performances of the prepared biosensors were also investigated.     

Determination of citrate released from stabilized gold nanoparticles by capillary zone electrophoresis

In this article, we report a pilot study on release of citrate ions from citrate-stabilized gold nanoparticles by capillary zone electrophoresis. First, a method for determination of citrate released from nanoparticles was developed using 5 mM phthalate buffer pH 6.5 with polybrene capillary inner wall coating. Within these conditions, citrate migrated in 1.8 min and it can be determined with LOD of 7 µmol/L using molybdate as the internal standard. The release of citrate was initiated by addition of common MOPS buffer ions used in capillary electrophoresis to nanoparticles samples. Non-linear behavior was found that proves release of citrate from stabilized gold nanoparticles. The release is initiated when 5 mM MOPS is added to nanoparticles’ solution. This behavior can partially explain zeta potential change of the nanoparticles from –34 mV in bulk solution to –28 mV in 50 mM MOPS.     

Electric detection of target DNA by fabricating gold nanowire bridges on planar nanogap electrodes

For the electrical detection of target DNA (partial avian influenza virus/H1N1/HA sequence) prepared via asymmetric PCR, we fabricated DNA-templated conducting gold nanowire bridges on planar nanogap electrodes using positively charged gold nanoparticles.     

Voltammetric behavior of gold nanotrench electrodes

We report the fabrication and electrochemical response of a gold nanoband electrode located at the bottom of a glass/epoxy nanotrench, hereafter referred to as a gold nanotrench electrode. Gold nanotrench electrodes of 12.5 and 40 nm in width with various depths from a few tens of nanometers to approximately 4 μm are fabricated and further characterized by cyclic voltammetry. The fabrication of a Au nanotrench electrode follows a simple electrochemical etching process in which a small AC signal is applied to an inlaid Au nanoband electrode submersed in a NaCl solution. The voltammetric behavior of a Au nanotrench electrode is characterized by a quasi-steady-state response at lower scan rates (e.g., <1 V/s for a 12.5-nm-wide electrode). We present an analytical expression for the quasi-steady-state diffusion-limited current of the nanotrench electrode based upon the analysis of the mass-transport resistance. Finite-element simulation of steady-state and transient voltammetric responses of the nanotrench electrodes provides additional insights for the analytical model. Peak-shaped transient voltammetric responses were observed at scan rates as low as 5 V/s for both inlaid and nanotrench electrodes. This result may suggest that the exposed area of the nanoband electrode is much greater than that expected from the fabrication of the inlaid bands. However, the extent to which this is seen is greatly decreased in the nanotrench electrode by a smoothing effect during etching. Our results confirm previous reports of excess overhanging metal and delamination crack contributing significantly to the shape and magnitude of the voltammetric response.     

DNA sequencing with titanium nitride electrodes

We construct a hydrogen‐bond based metal–molecule–metal junction, which contains two identical “reader” molecules, one single DNA base as a bridged molecule, and two titanium nitride electrodes. Hydrogen bonds are formed between “reader” molecules and DNA base, whereas titanium–sulfur bonds are formed between “reader” molecules and titanium nitride electrodes. We perform electronic structure calculations for both the bare bridged molecule and the full metal–molecule–metal system. The projected density of states shows that when the molecule is connected to the titanium nitride electrode, the energy levels of the bridged molecule are shifted, with an indirect effect on the hydrogen bonds. This is similar to the case for a gold electrode but with a more pronounced effect. We also calculate the current–voltage characteristics for the molecular junctions containing each DNA base. Results show that titanium nitride as an electrode can generate distinct conductance for each DNA base, providing an alternative electrode for DNA sequencing. © 2013 Wiley Periodicals, Inc.     

Underpotential deposition of copper on electrodes modified with colloidal gold

This communication is concerned with the electrochemical addressability of gold colloidal particles deposited on a conducting substrate. Cyclic voltammetry of electrodes modified with gold colloid layers indicates that an underpotential deposition of copper onto the gold surface takes place. Analysis of the charge associated with the underpotential deposition permits the electroactive gold area to be calculated. The total gold area may be determined from transmission electron microscopy (TEM) images. Comparison of the geometric and electroactive areas obtained indicates that electrochemically all the gold particles are addressable and the entire colloid surface is accessible.     

Voltammetric determination of DNA hybridization using methylene blue and self-assembled alkanethiol monolayer on gold electrodes

An electrochemical DNA biosensor based on the recognition of single stranded DNA (ssDNA) by hybridization detection with immobilized complementary DNA oligonucleotides is presented. DNA and oligonucleotides were covalently attached through free amines on the DNA bases using N-hydroxysulfosuccinimide (NHS) and N-(3-dimethylamino)propyl-N′-ethylcarbodiimide hydrochloride (EDC) onto a carboxylate terminated alkanethiol self-assembled monolayers (SAM) preformed on a gold electrode (AuE). Differential pulse voltammetry (DPV) was used to investigate the surface coverage and molecular orientation of the immobilized DNA molecules. The covalently immobilized probe could selectively hybridize with the target DNA to form a hybrid on the surface despite the bases being attached to the SAM. The changes in the peak currents of methylene blue (MB), an electroactive label, were observed upon hybridization of probe with the target. Peak currents were found to increase in the following order: hybrid-modified AuE, mismatched hybrid-modified AuE, and the probe-modified AuE which indicates the MB signal is determined by the extent of exposed bases. Control experiments were performed using a non-complementary DNA sequence. The effect of the DNA target concentration on the hybridization signal was also studied. The interaction of MB with inosine substituted probes was investigated. Performance characteristics of the sensor are described.     

Analytical applications of gold electrodes modified with monolayers of thiolated cyclodextrins

The sequential method for the preparation of cyclodextrin monolayers is used to prepare modified electrodes responsive towards selected guest molecules: ferrocene, ibuprofen, methylene blue, dopamine and menadione. The inclusion into cyclodextrin cavities is monitored using cyclic voltammetry and the mediating role of the immobilized molecules towards solution species is shown on the example of dopamine oxidation.     

An extraordinary electrocatalytic reduction of oxygen on gold nanoparticles-electrodeposited gold electrodes

The cathodic reduction of oxygen has been investigated at a gold nanoparticles-electrodeposited gold electrode in 0.5 M H₂SO₄ solution. Two well-defined reduction peaks were observed at +50 and −250 mV vs. Ag/AgCl/KCl (sat.). Those two peaks indicated a 2-step 4-electron reduction pathway of O₂ in this strong acidic medium. The former peak was ascribable to the 2-electron reduction of O₂ to H₂O₂, while the latter was assigned to the reduction of H₂O₂ to H₂O. The observed electrocatalysis for the reduction of O₂ is attributable to the extraordinary catalytic activity of the gold nanoparticles over the bulk gold electrode, at which the 2-electron reduction peak of O₂ to H₂O₂ was observed at −200 mV.     

Facile fabrication of nanoporous gold film electrodes

A novel one-step method has been developed for the fabrication of a three-dimensional (3D) nanoporous gold film (NPGF). The NPGF can be facilely made within 1 min from a pure gold substrate by applying a step potential just into the initial transition region of gold in an HCl medium. The pore formation and structural evolution have been revealed by scanning electron microscope, and the processes involve electrodissolution, disproportion, and deposition. The as-prepared 3D NPGF electrode has a large surface area and exhibits high catalytic activity in the electrooxidation of glucose. The NPGF electrode also shows excellent performance toward the electrooxidation of formic acid after being decorated with a tiny amount of Pt by electrodeposition.     

On-demand electrochemical release of DNA from gold surfaces

Electrochemical quartz crystal microbalance (EQCM) is used to probe the electrochemically triggered release of nucleic acids from gold surfaces to solutions of physiological pH. The immobilization of nonthiolated DNA onto the gold surface is followed by an electrostatic desorption at -1.0 V (vs. Ag/AgCl). Steady-state frequency signals, corresponding to the removal of 261- and 644-ng/cm2 single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA), respectively, were attained within 75 and 330 s. As expected for electrostatic repulsion, the amount released can be manipulated by tuning the potential. The small nonsteady-state frequency signals observed at lower potentials indicate promise for a sustained DNA release. Applicability to gold ultramicroelectrodes (12.5-microm radius) is demonstrated in connection with voltammetric blocking experiments. We expect that such on-demand electrochemical release would be a useful addition to the arsenal of nonviral gene delivery routes.     

A study of lithium insertion into electrodes with thin gold films

The process of lithium insertion into thin gold films is studied. It is found that lithium is reversibly inserted with the formation of compounds, which are close to Li_1.5Au in their average composition. The long-term cycling leads to the breakdown of gold layer (and corresponding decrease in the electrode capacity), which is associated with considerable volume changes due to lithium insertion.