Coated Gold Electrode Research Articles

An ultrasensitive protein array based on electrochemical enzyme immunoassay

An ultrasensitive electrochemical enzyme immunoassay (EEIA) for the detection of proteins on an 8x8 array is described. The assay is based on wired enzyme technology. Briefly, capture antibody was covalently immobilized on a self-assembled 11-mercaptoundecanoic acid (MUA) monolayer coated gold electrode. After incubating with a target protein (antigen), the gold electrode was treated sequentially in biotinylated detection antibody solution and in avidin-horseradish peroxidase conjugate (A-HRP) solution. A cationic redox polymer (electrochemical mediator) overcoating was applied to the gold electrode through layer-by-layer electrostatic self-assembly. The formation of a bilayer brought the HRP in electrical contact with the underlying electrode, making the bilayer an electrocatalyst for the reduction of hydrogen peroxide where the redox polymer acts as an artificial mediator. Consequently, the concentration of protein could be quantified amperometrically. This electrochemical immunoassay combined the specificity of the immunological reaction with the sensitivity of the electrochemical detection. The applicability of the system in protein detection was demonstrated with a snake toxin, beta-bungarotoxin, a neurotoxin from the venom of the snake Bungarus multicinctues. Under optimized experimental conditions, the assay allowed the detection of beta-bungarotoxin in the range of 20 pg/mL to 1.5 ng/mL with a detection limit of 10 pg/mL (20 fg). A higher detection limit of 25 pg/mL was obtained in serum.

A study of the hydrophobic properties of alkanethiol self-assembled monolayers prepared in different solvents

We have measured the interfacial capacitance of an alkanethiol self-assembled monolayer (SAM) and its barrier properties adsorbed in different non-aqueous solvents by means of electrochemical impedance spectroscopy and cyclic voltammetry. Our impedance results show that the permeability of the SAM depends to a large extent on the solvent used as the adsorption medium. Monolayers of alkanethiol are highly impermeable when formed in solvents like hexane and chloroform, compared to those formed in solvents like ethanol, DMF, acetonitrile, hexadecane and toluene. Our studies lead us to believe that a thin hydrophobic layer separates the alkanethiol SAM and the aqueous electrolyte when the monolayer is prepared in hexane and chloroform. This layer acts as a series capacitor to the dielectric film of the alkanethiol monolayer. This tends to lower the effective interfacial capacitance and also enhances the uncompensated solution resistance of the monolayer coated gold electrode. Our experimental results may indicate a large-scale depletion of water density as predicted by the theory of Lum Chandler and Weeks (LCW theory, J. Phys. Chem. B 103 (1999) 4570), or the presence of nanobubbles or cavities at the highly hydrophobic interface.

Toluidine blue modified self-assembled silica gel coated gold electrode as biosensor for NADH

A toluidine blue modified gold electrode was constructed using self-assembled silica gel technique. Firstly, toluidine blue was encapsulated within 3D network of silica self-assembly monolayer on the surface of gold electrode. Secondly, another layer of silica sol was further assembled to protect from leaching of mediator or possible contamination. The electrochemical characteristics of toluidine blue immobilized within self-assembled silica gel were studied in detail. The modified electrode was applied for electrochemical oxidation of NADH with satisfactory results.

Kinetics of reduction of nitrobenzene and carbon tetrachloride at an iron-oxide coated gold electrode.

The rates of reduction of carbon tetrachloride (CT) and nitrobenzene (NB) by iron-oxide coated gold electrodes were studied to gain insight into the processes that control reduction of groundwater contaminants by zerovalent metal permeable reactive barriers. Fe(III)-oxide films were deposited on gold electrodes with a small fraction of the Fe(III) electrochemically reduced to Fe(II) to investigate the role of Fe(II) in the reduction of the CT and NB. Mass transport to the surface of the oxide film was controlled through use of a well-defined flow-through system similar to a wall-jet electrode. The factors affecting the overall reduction rate were investigated by varying the Fe(II) content in the iron-oxide, controlling mass transport of the electroactive species to the oxide surface, and varying the thickness of the oxide film. The rates of reduction of CT and NB were found to be independent of Fe(II) content in the iron-oxide and were only slightly dependent on the rate of transport to the surface of the oxide under a few sets of reaction conditions. Conversely, the rates of reduction were greatly dependent on the thickness of the oxide film, with the reduction rate decreasing as the oxide thickness increased. Evidence suggests that the location of the reduction reaction for CT and NB is at the gold surface and supports a barrier model for the system studied, in which the oxide film physically impedes direct contact of the electroactive species and the gold electrode, increases the diffusion path length, and creates adsorption sites.

Electrochemical Properties of a Thiol Monolayers Coated Gold Electrode Modified with Osmium Gel Membrane as Enzyme Sensor

The electrochemical behavior of an enzyme sensor for glucose using a gold electrode modified with thiol self-assembled membrane and osmium complex gel as an electron transferring mediator has further been investigated by electrochemical analysis. The gold electrode was initially coated with aminomethanethiol self assembling mono layer membrane(thiol-SAM) and then immobilized with glucose oxidase using poly(vinylpyridine-co-allylamine) (PVP-co-AA), gel coordinated with osmium bipyridine complexes (GOD/Os-PVP-co-AA gel). The cleaning condition of the surface of the Au electrode prior to coating thiol SAM was optimized for reduction of interference caused by concomitant compounds. It was found that interfering influence was most efficiently reduced in the case of use of the Au electrode immersed into nitric acid. The current ratio with a thiol coated gold electrode modified with Os-PVP-co-AA gel in glucose solution in the presence to absence of ascorbic acid, acetaminophen, and uric acid ( I D+I / I I ) was 1.006, 1.014, and 1.018, respectively. The peak current response of glucose in the electrode modified with thiol SAM was dropped to 60 – 98% as compared with that without thiol SAM.

Self-assembly of (3-mercaptopropyl)trimethoxysilane on iodine coated gold electrodes

The adsorption of (3-mercaptopropyl)trimethoxysilane (MPS) has been studied on iodine coated gold electrodes. The MPS adsorption from alcoholic solution on Au(111) and iodine coated Au(111) was studied by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrochemical formation of MPS monolayers was studied by cyclic voltammetry on polycrystalline uncoated and coated gold electrodes with different MPS pre-treatment conditions. Lead electrochemical deposition was used to probe the defect sites of the surfaces created. The MPS-over-iodine coated gold surface produces a lower-density monolayer than the MPS over pure gold. The MPS monolayer formed electrochemically on the iodine coated gold is chemically equal to its counterpart after the iodine desorption. The MPS adsorption occurs via an AuS bond, after the partial reductive-desorption of the iodine monolayer from the iodine coated gold electrode, and produces an ordered composite monolayer of MPS/iodine. The size of the defects can be controlled by varying the electrochemical preparation conditions, using the following reaction: AuI (ads)+MPSH (ac)+e −↔AuMPS (ads)+H (ac) ++I (ac) −.

Rotating ring-disk electrode study on the cathodic oxygen reduction at Nafion®-coated gold electrodes

Abstract The influence of Nafion® films on the kinetics of cathodic oxygen reduction was investigated using Nafion®-coated gold electrodes. A rotating ring-disk electrode (RRDE) technique was used to determine kinetic parameters for O2 reduction and to detect intermediate H2O2 formation. Kinetic parameters such as the kinetic current, Tafel slope, potential dependence of the ratio of the direct 4-electron reduction to the series 2-electron reduction were determined and compared with those obtained at bare Au in 0.5 M H2SO4. The kinetic data showed no clear evidence of the enhancement effects of Nafion®-coating on the cathodic O2 reduction at Au. This suggested that the Au surface has less affinity for the hydrophobic region of Nafion®, in which a high O2 solubility is expected.

Electrochemical deposition of poly(o-anisidine) and polypyrrole at octadecanethiol coated gold electrodes

The o-anisidine and pyrrole have been polymerized by the electrochemical oxidation of monomers on gold electrodes, covered with self-assembled monolayers. The obtained polymer–monolayer systems have been investigated by cyclic voltammetry in aqueous supporting electrolyte solutions containing K4Fe(CN)6 and Ru(NH3)6Cl3. The deposition of conducting polymers strongly depends on the integrity of a monolayer. In the case of a large number of SAM defects, the polymerization of o-anisidine and pyrrole leads to the formation of nuclei of the conducting polymer in the insulating matrix of the thiol monolayer. When the polymer is in the conducting (oxidized) form, the nuclei act as an array of microelectrodes. The polarogram-shaped voltammograms obtained for K4Fe(CN)6 confirm the hemispherical diffusion of redox species to the polymer nuclei. When the polymer is in the non-conducting (reduced) form, the polymer–octadecanethiol layer blocks the redox processes on the electrode. The exponential-type CV curves observed for Ru(NH3)6Cl3, when the polymer is in its non-conducting state, can be assigned to the tunnelling of electrons through the passivating layer. The use of monolayers with a low number of defects influences the mechanism of polymer growth. Thus, the polypyrrole grows on the layer of thiols, and the poly(o-anisidine) forms polymer nuclei.

Design of novel molecular wires for realizing long-distance electron transfer

Novel heteroarene oligomers, consisting of two pyridinium groups, linked by thiophene units of variable length, “thienoviologens”, are described as promising candidates for molecular wires. Two representative thienoviologens were coated by adsorption from micromolar concentrations in ethanol onto octadecylmercaptan (ODM)-coated gold electrodes and induced a gradual restoration of the electrochemistry with hexacyanoferrate as a function of molecular wire concentration. Glucose oxidase and choline oxidase showed strong adsorption to these conductive layers, but showed striking differences in adsorption to the different thienoviologen layers. The measurements support the hypothesis that the molecules are incorporated in the ODM layer in a different fashion. Also the complex formation of an engineered azurin redox protein with water-soluble pyridyl ligands is presented in relation to a possible application of the thienoviologens as conductive spacers, in which the contact with the redox protein is achieved via complex formation with a free pyridine nitrogen.

Analytical application of self assembled monolayers on gold electrodes: critical importance of surface pretreatment

Polycrystalline bare gold electrodes have been used as substrate for the preparation of self assembled monolayers (SAMs) of alkanethiols. Chemical cleaning and several mechanical polishing procedures of the electrode have been carefully tested in order to prepare monolayers of octadecyl-, mercaptans. By analyzing the cyclic voltammetric curves of hexacyanoferrate III and chlorpromazine and the electrochemical desorption of the coated monolayer, it appeared that the response at the bare electrodes is related to the cleaning procedure and that the structure of SAM coated gold electrodes is influenced by the polishing material used, i.e. diamond or alumina slurry. Electrochemical results have been confirmed by Auger analysis of the electrode surface.

Covalent binding of viologen to electrode surfaces coated with poly(acrylic acid) prepared by electropolymerization of acrylate ions: II. Effect of the ionization state of the polymeric coating on the formal potential of viologen

The contribution of carboxy groups of poly(acrylic acid) (P(AA)) to the electrochemical properties of soluble and linked viologen molecules was studied by cyclic voltammetry. P(AA)-coated gold electrodes showed a pH-dependent selectivity towards anionic (hexacyanoferrate) or cationic (methyl viologen) redox compounds. A formal redox potential of −0.62 V (vs. saturated calomel electrode), independent of pH, was found for aminopropyl methyl viologen (MAV) in solution. However, the redox potential of this viologen covalently linked to P(AA) showed a complex dependence on the pH: it was constant when the pH was lower than 4.2 or higher than 6.5, while there was a shift between these two negative potentials when the pH varied from 4.5 to 6.5. This pH dependence was suppressed after esterification or amidation of the free carboxy groups of P(AA). The redox potential of viologen bound to P(AA) changed with the loading of viologen. Redox potentials of soluble and covalently linked viologen showed a different dependence on NACl concentration at pH 7.5. The electrochernical behaviour of viologen bound to P(AA) is attributed to a change in the intrinsic oxidizing or reducing strength of the MAV .+/MAV 2+ pair as a consequence of its incorporation with P(AA). The possibility of tuning the redox potential of the immobilized viologen in a controlled way is demonstrated and its application to enzyme electrodes discussed.

Development of an enzyme-based biosensor for atrazine detection

An amperometric biosensor for detection of the photosynthetic inhibiting herbicide atrazine is described. The sensor, incorporating the enzyme tyrosinase, was sensitive to a wide range of di- and triphenols. It displayed a fast response, with 95% of the steady-state current being obtained within 25 s, and had a half-life of 8 d but retained some activity for up to 20 d. This biosensor displayed a notable decrease in response in the presence of atrazine. Atrazine inhibition of the biosensor response was found to be reversible. The electrode construction consisted of tyrosinase cross-linked on a poly(pyrrole) coated gold electrode surface. The inhibition of enzyme activity in the presence of atrazine was measured amperometrically. Atrazine concentrations of 5 × 10–6 mol dm–3 were measured in a hydrodynamic cell and of 1 × 10–5 mol dm–3 in a flow-through system.

Amperometric determination of carbon dioxide using a gold electrode modified by the underpotential‐deposited cadmium

AbstractAn underpotential‐deposited cadmium layer formed on a gold electrode was used to amperometrically determine PCO2 in an aqueous medium. This electrode reduces nitrate anions in an aqueous solution. The reduction current produced can be related to the proton concentration. Thus, when dissolved CO2 is present in a nitrate solution it alters the proton concentration, which can then be measured amperometrically. Experimental investigation of this system and a Nafion coated gold electrode were both carried out. The effect of oxygen present in the system is also discussed.

Microporous aluminum oxide films at electrodes part II. studies of electron transport in the Al 2O 3 matrix derivatized by adsorption of poly(4-vinylpyridine)

Porous aluminum oxide films are used as matrices for immobilization of electroactive reagents at electrodes. Aluminum oxide of highly regular porosity is produced by electrooxidation of aluminum substrates in polyprotic acid electrolytes. Its porosity consists of densely packed cylindrical pores perpendicular to the oxide film and penetrating its entire thickness. Depending on the applied voltage of the preparatory electrolysis, the average pore diameter of the oxide film can be varied in the range of ca. 10 nm to 200 nm. After separation of the Al 2O 3 film from the aluminum substrate and the appropriate chemical tratment, the oxide films are overcoated with gold in vacuum in order to prepare porous aluminum oxide coated gold electrodes. The porosity of the Al 2O 3 films ranges from 38% to 55% as assessed by rotating disk voltametry and transmission electron microscopy. Adsorption fo poly(4-vinylpyridine) along the pores of the oxide film results in binding of iron hexacyanide ions from acid media. The rate of electron transport along the pores of the oxide film was studied by a transient method, chronocoulometry and by a steady state technique, rotating disk voltammetry. Due to the heterogeneous nature of the electrode films in which electroactive ions are simultaneously present in the polymer phase and the aqueous phase of the oxide pores, the electron transport involves charge diffusion in both phases coupled by rapid electron and mass exchange equilibria. A new and generally applicable method of chronocoulometric data analysis was developed which allowed calculation of the diffusion coefficient of electron transport in the polymer phase, D p. This data analysis method parallels that developed by Anson et al. for steady-state conditions (J. Phys. Chem., 87 (1983) 214). Both techniques gave a D p value of ca. 1.0 × 10 −8 cm 2/s.