EQCM and Fluoroelectrochemical Studies on the Catalytic Oxidation of NADH at a Pencil 8B‐Scrawled Gold Electrode with High Detection Sensitivity

We report for the first time the effective catalytic electrooxidation of nicotinamide adenine dinucleotide (NADH) on the pencil 8B‐scrawled gold electrode of an electrochemical quartz crystal microbalance (EQCM). The EQCM allowed us to quantitatively evaluate the catalytic activity of the pencil‐scrawled Au electrode. With increasing the mass of modified pencil powders, the peak potential for NADH oxidation shifted negatively, with maximum shift of ?0.35 V at saturated pencil modification; the NADH‐oxidation peak current density (j_p) was also notably increased, and the j_p at saturated pencil modification was found to be larger than those at conventional pencil 8B and bare Au electrodes. Sensitive amperometric detection of NADH was achieved at the gold electrode with saturated pencil modification, with low detection potential (0.4 V versus SCE), low detection limit (0.08 μmol L^?1) and wide linear range (0.2–710 μmol L^?1). The fluoroelectrochemical measurements of NADH at bare and pencil‐modified gold electrodes were also conducted with satisfactory results. The convenient and low‐cost modification of pencil powders on the Au electrode may have presented a new functional surface of the EQCM, which is recommended for wider applications to bioelectrochemical studies, especially in view of the EQCM's capability of providing abundant in situ information in relevant processes.     

Quartz crystal microbalance measurements of kinetic parameters of anodic dissolution of gold in thiocarbamide electrolytes in the presence of sulfide ions

As shown by quartz-crystal microbalance measurements, in the potential range from 0.0 to 0.55 V (NHE), sulfide ions adsorbed on the gold electrode surface accelerate the electrode reaction of anodic dissolution of gold in acidic thiocarbamide solutions. The microbalance determination of kinetic parameters at a constant electrode surface coverage with sulfide ions includes a special procedure developed for the determination of the gold dissolution rate. The conditions (the potential range and the potential scan rate) of independence of the dissolution rate from the diffusion limitations associated with the ligand delivery is determined. Under these conditions, the polarization curve is shown to be linear on semilogarithmic coordinates and correspond to the Tafel equation. In this potential range, the transfer coefficient α and the reaction order with respect to the ligand p are determined at a constant electrode surface coverage θ with adsorbed sulfide ions. It is shown that with the transition from the surface coverage with sulfide ions θ = 0.1 to θ = 0.8, the transfer coefficient α changes from 0.25 to 0.55, the exchange current (i ₀) changes from 10^?5 to 5 × 10^?5 A/cm², and the effective reaction order p with respect to the ligand changes from 0.2 to 1.3. The mentioned changes are associated not only with the acceleration of gold dissolution in the presence of chemisorbed sulfide ions but also with the changeover in the mechanism of this process. Quartz-crystal microbalance data on the gold dissolution rate qualitatively agree with the results of voltammetric measurements of a renewable gold electrode. A possible version of explanation of the catalytic effect of sulfide ion adsorption on the gold dissolution is put forward.     

Microsensor Chip Integrated with Gold Nanoparticles‐Modified Ultramicroelectrode Array for Improved Electroanalytical Measurement of Copper Ions

This paper presents a microsensor chip integrated with a gold nanoparticles‐modified ultramicroelectrode array (UMEA) as the working electrode for the detection of copper ions in water. The microsensor chip was fabricated with Micro‐Electromechanical System technique. Gold nanoparticles were electrodeposited onto the surface of UMEA at a constant potential of ?0.3 V. The ratio d/R_b of interelectrode spacing (d) over the individual electrode’s radius (R_b) was investigated to improve the electrochemical performance. The UMEA with a d/R_b of 20 showed the best hemispherical diffusion mode, resulted in fast response time and high current response. The gold nanoparticles increased the active surface area of UMEA by not changing the geometries of UMEA, and the current response was increased further. Incorporating the optimized characteristic of UMEA and gold nanoparticles, the microsensor showed a good linear range from 0.5 to 200 µg L^?1 of copper ions in the acetate buffer solutions with the method of square wave stripping voltammetry. Compared with the gold nanoparticles‐modified disk electrode, the gold nanoparticles‐modified UMEA showed higher sensitivity (0.024 µA mm^?2 µg^?1 L) and lower limit of detection (0.2 µg L^?1). Water samples from river water and tap water were analyzed by the microsensor chip with recovery ranging from 100.7 % to 107.8 %.     

A Simple and Highly Stable Porous Gold‐based Electrochemical Sensor for Bisphenol A Detection

A porous gold electrode with a uniform pore size was prepared by a simple single potential step method. It provided around 19 times more surface area than a bare gold electrode. The porous gold electrode, without any modification, exhibited good electrocatalytic activity towards the oxidation of bisphenol A (BPA). The effect of the pH and scan rate, on the porous gold electrode was studied and discussed. Under optimum conditions of the flow system, the calibration plot of BPA was linear over a wide range of concentration, from 2.0 nM to 800 nM (R²=0.999), and the detection limit was 2.0 nM (s/n=3). It was of interest that, the porous gold electrode showed an exceptional result in minimizing the fouling from the oxidation product of BPA and led to a very high operational stability of the electrode for the detection of BPA (560 consecutive times). The porous gold electrode showed good fabrication reproducibility and can be used to detect bisphenol A that leached from baby bottles and drinking water bottles. The obtained results were in good agreement with the GC‐MS method (P >0.05).     

Kinetics and mechanism of gold dissolution in thiourea solutions: Effect of sulfide ions

Chronoamperograms for gold in solutions containing 0.1 M thiourea, 0.5 M H₂SO₄, and catalytically active sulfide ions at the concentration c ₁ from 1 × 10^?5 to 4 × 10^?5 M are obtained at different potentials with the aid of an automated setup intended for renewing the electrode surface directly in the solution by cutting off a thin surface layer of the metal. It is shown that the results of measurements of the current practically coincide at a constant value of the product c ₁ t, where t is the time period elapsed after the renewal of the electrode surface. Such a coincidence testifies to a diffusion nature of processes that hamper accumulation of sulfide ions at the gold surface. This fact permitted the use of a procedure developed previously for the calculation of polarization curves at constant values of surface coverage θ by catalytically active ions. At θ = const, the voltammetric curves for gold in sulfide-containing thiourea solutions are shown to correspond to the Tafel equation. With the surface coverage increasing, the effective values of the exchange current i ₀, transfer coefficient α, and anodic reaction order with respect to thiourea P _a increase from the values i }~ 10^?5 A cm^?2, α }~ 0.12, and P _a = 0.2, which are characteristic of pure solutions, to 2 × 10^?4 A cm^?2, α }~ 0.5, and P _a = 1.1 (at θ }~ 0.5). An interpretation to the established regularities is given.     

Osmium Redox Polymer Film Electrode and Its Electrocatalytic Properties towards the Oxidation of Epinephrine: Electrochemical Quartz-Crystal Microbalance and Voltametric Characterization

An osmium redox polymer PVI-PAA-dmeOs is electrodeposited onto a gold electrode by using repetitive double potential step chronoamperometry. The resulting film is permeable to the substrates and products of the catalyzed reaction, and permits fast electron transfer. The frequency variation during the potential step process is recorded using electrochemical quartz-crystal microbalance (EQCM). A reaction mechanism for electrochemical deposition of the osmium redox polymer is proposed. The characteristics of the PVI-PAA-dmeOs film are investigated by EQCM and cyclic voltametry. The results show the hydrated osmium PVI-PAA-dmeOs film to exhibit excellent electrocatalytic activity towards the oxidation of epinephrine. At a bare gold electrode, the epinephrine oxidation current increases greatly and the oxidation peak potential negatively shifts to about 0.16 V (Ag/AgCl) at the film-modified electrode. Under optimal conditions, amperometric measurements are performed at 0.18 V and the current response of epinephrine changes linearly with its concentration from 5 × 10^?7 to 1 × 10^?4 mol l^?1. A detection limit of 1.5 × 10^?7 mol l^?1 (S/N = 3) is obtained.     

Quartz crystal microbalance monitoring of density changes in mesoporous TiO2 phytate films during redox and ion exchange processes

Nanofilm deposits of TiO₂ nanoparticle phytates are formed on gold electrode surfaces by ‘directed assembly’ methods. Alternate exposure of a 3-mercapto-propionic acid modified gold surface to (i) a TiO₂ sol and (ii) an aqueous phytic acid solution (pH 3) results in layer-by-layer formation of a mesoporous film. Ru(NH₃)₆³⁺ is shown to strongly adsorb/accumulate into the mesoporous structure whilst remaining electrochemically active. Scanning the electrode potential into a sufficiently negative potential range allows the Ru(NH₃)₆³⁺ complex to be reduced to Ru(NH₃)₆²⁺ which undergoes immediate desorption. When applied to a gold coated quartz crystal microbalance (QCM) sensor, electrochemically driven adsorption and desorption processes in the mesoporous structure become directly detectable as a frequency response, which corresponds directly to a mass or density change in the membrane. The frequency response (at least for thin films) is proportional to the thickness of the mass-responsive film, which suggests good mechanical coupling between electrode and film. Based on this observation, a method for the amplified QCM detection of small mass/density changes is proposed by conducting measurements in rigid mesoporous structures.     

Electrochemically induced transformations of heteropolyanions: new electroactive metal oxide films studied by the electrochemical quartz crystal microbalance

New oxide films have been electrodeposited from [P₂Mo₁₈O₆₂]⁶⁻ by potential cycling in mildly acidic aqueous media. To obtain an adherent and persistent film, it is necessary that more than six electrons/molecule be fixed on the framework of the heteropolyanion. The film is then studied in pure supporting electrolyte. In this medium, a remarkable current increase is observed during the potential cycling. Whether the film is deposited on a glassy carbon electrode or on the gold electrode of an electrochemical quartz crystal microbalance (EQCM), exactly the same steady current increase up to a maximum is obtained in cyclic voltammetric measurements. The EQCM reveals a steady mass increase during the continuous cycling of the film in the supporting electrolyte. This behaviour is interpreted as featuring an irreversible water and electrolyte intake into the film, up to a maximum, after which the phenomena observed during reduction and oxidation processes are taken as featuring intercalation/deintercalation, respectively. This behaviour is much the same as described in the literature for WO₃ and MoO₃ bronzes, except that the present films seem very stable and have shown no tendency to dissolve or deactivate. Received: 2 December 1998 / Accepted: 26 January 1999     

Catalytic Effect on Silver Electrodeposition of Gold Deposited on Carbon Electrodes

A new methodology, based on silver electrocatalytic deposition and designed to quantify gold deposited onto carbon paste electrode (CPE) and glassy carbon electrode (GCE), has been developed in this work. Silver (prepared in 1.0 M NH₃) electrodeposition at ?0.13 V occurs only when gold is previously deposited at an adequate potential on the electrode surface for a fixed period of time. When a CPE is used as working electrode, an adequate oxidation of gold is necessary. This oxidation is carried out in both 0.1 M NaOH and 0.1 M H₂SO₄ at oxidation potentials. When a GCE is used as working electrode, the oxidation steps are not necessary. Moreover, a cleaning step in KCN, which removes gold from electrode surface, is included. To obtain reproducibility in the analytical signal, the surface of the electrodes must be suitably pretreated; this electrodic pretreatment depends on the kind of electrode used as working electrode. Low detection limits (5.0×10^?10 M) for short gold deposition times (10 min for CPE and 5 min for GCE) were achieved with this novel methodology. Finally, sodium aurothiomalate can be quantified using silver electrocatalytic deposition and GCE as working electrode. Good linear relationship between silver anodic stripping peak and aurothiomalate concentration was found from 5.0×10^?10 M to 1.0×10^?8 M.     

Preparation of a Novel NH2 + Ion Implantation Modified Electrode and Its Study of the Electrochemical Behavior of Hemoglobin

Abstract

A novel functional electrode was obtained by implanting NH₂ ⁺ into ITO film (NH₂/ITO) for the first time. The NH₂/ITO surface showed a better affinity to gold nanoparticles than bare ITO. Gold nanoparticles were deposited on the surface of NH₂/ITO electrode (Au/NH₂/ITO). The Au/NH₂/ITO and NH₂/ITO electrodes were used to observe the electrochemical behavior of Hemoglobin (Hb) immobilized on the electrodes surfaces. The peak current value of Hb immobilized on NH₂/ITO increased compared with on bare ITO while peak current value of Hb immobilized on Au/NH₂/ITO increased compared with on Au/ITO. Linkage between the ‐NH₂ implanted into the ITO film and the ‐COOH of Hb was thought to be the reason for the increase of active Hb coverage on NH₂/ITO compared with bare ITO. Increase of active Hb coverage on Au/NH₂/ITO compare with Au/ITO was attributed to the different amount of gold nanoparticles deposited. Results showed the novel NH₂/ITO and Au/NH₂/ITO electrodes exhibited good stability, reproducibility besides selectivity and sensitivity. The electrode process of Hb immobilized on Au/NH₂/ITO was quasi‐reversible with adsorption. The electrode reaction rate constant k_s and other related constants were determined. X‐ray photoelectron spectroscopy (XPS), field‐emission scanning electron microscopy (FE‐SEM), and impedance spectra were used to characterize the different surfaces.     

Surface poisoning during electrocatalytic monosaccharide oxidation reactions at gold electrodes in alkaline medium

In the present study, the surface poisoning of electrocatalytic monosaccharide oxidation reactions at gold electrodes were investigated. In the cyclic voltammetric studies, the electrocatalytic oxidation of aldohexose and aldopentose type monosaccharides, aminosugars, acetyl-glucosamine and glucronamide were observed at gold plate electrodes in alkaline medium. However, in controlled-potential electrolytic studies ranging −0.3 to −0.2 V in reaction solutions, current flows during electrolyses decreased quickly with time, except when glucosamine was used as a substrate.Results from surface enhanced infrared adsorption (SEIRA) spectroscopic measurements at an evaporated gold electrode for the electrocatalytic oxidation of glucose in 0.1 mol dm⁻³ NaOH at −0.3 V and Gaussian simulated spectra indicated that the gluconic acid as a 2-electron oxidation product and/or its analogs adsorbed onto the gold surface. Electrochemical quartz crystal microbalance (EQCM) measurement results, along with surface adsorption results from surface poisoning at the gold electrode during electrolytic reactions, suggested that gluconic acid and/or its analogs adsorbed vertically onto electrode surfaces in a full monolayer packing-like conformation. In the case of the electro oxidation of glucosamine in 0.1 mol dm⁻³ NaOH at −0.2 V, the obtained SEIRA spectra and EQCM results, clearly indicated that the glucosaminic acid as a 2-oxidation glucosamine product did not strongly bind onto the gold electrode surface.     

Electrochemical immunosensor based on nanoporpus gold loading thionine for carcinoembryonic antigen

Nanoporous gold (NPG) has recently received considerable attention in analytical electrochemistry because of its good conductivity and large specific surface area. A facile layer-by-layer assembly technique fabricated NPG was used to construct an electrochemical immunosensor for carcinoembryonic antigen (CEA). NPG was fabricated on glassy carbon (GC) electrode by alternatively assembling gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) using 1,4-benzenedimethanethiol as a cross-linker, and then AgNPs were dissolved with HNO₃. The thionine was absorbed into the NPG and then gold nanostructure was electrodeposited on the surface through the electrochemical reduction of gold chloride tetrahydrate (HAuCl₄). The anti-CEA was directly adsorbed on gold nanostructure fixed on the GC electrode. The linear range of the immunosensor was from 10 pg mL⁻¹ to 100 ng mL⁻¹ with a detection limit of 3 pg mL⁻¹ (S/N = 3). The proposed immunosensor has high sensitivity, wide linear range, low detection limit, and good selectivity. The present method could be widely applied to construct other immunosensors.     

A carbon electrode sputtered with palladium and gold for the amperometric detection of hydrogen peroxide

A modified carbon electrode for the amperometric determination of hydrogen peroxide is described. By deposition of a 15-nm thick layer of a 40:60 mixture of palladium and gold on the surface of the electrode the overvoltages for both the oxidation and the reduction can be decreased by at least 800 mV. When applied as an electrochemical sensor in a flow-injection system, linear calibration graphs were obtained between 10^?7 and 5 × 10^?3 M hydrogen peroxide. The modified electrodes were stable for months.     

Electrochemical quartz crystal microbalance study of perchlorate and perrhenate anion adsorption on polycrystalline gold electrode

The electrochemical quartz crystal microbalance (EQCM) was used to study adsorption/desorption of perchlorate and perrhenate ions on a bare polycrystalline gold electrode. An electrode mass change in perrhenate solution was about double that of perchlorate. The equivalent mass of adsorbed anions (about 260 and 120 g F⁻¹ respectively) suggests adsorption of perchlorate and perrhenate anions on a polycrystalline gold electrode in the double-layer region. Water molecules are partially expelled from the gold surface during the initial stages of anion adsorption. The water loss is about three times larger for perrhenate compared to perchlorate due to the bigger ionic radius (volume) of the perrhenate anion.     

Bioelectrocatalysis by Microperoxidase‐11 in a Multilayer Architecture of Chitosan Embedded Gold Nanoparticles

We report on the redox behaviour of the microperoxidase‐11 (MP‐11) which has been electrostatically immobilized in a matrix of chitosan‐embedded gold nanoparticles on the surface of a glassy carbon electrode. MP‐11 contains a covalently bound heme c as the redox active group that exchanges electrons with the electrode via the gold nanoparticles. Electroactive surface concentration of MP‐11 at high scan rate is between 350±50 pmol cm^?2, which reflects a multilayer process. The formal potential (E°′) of MP‐11 in the gold nanoparticles‐chitosan film was estimated to be ?(267.7±2.9) mV at pH 7.0. The heterogeneous electron transfer rate constant (k_s) starts at 1.21 s^?1 and levels off at 6.45 s^?1 in the scan rate range from 0.1 to 2.0 V s^?1. Oxidation and reduction of MP‐11 by hydrogen peroxide and superoxide, respectively have been coupled to the direct electron transfer of MP‐11.     

Construction of glucose biosensor based on sorption of glucose oxidase onto multilayers of polyelectrolyte/nanoparticles

A new approach to constructing an enzyme-containing film on the surface of a gold electrode for use as a biosensor is described. A basic multilayer film (BMF) of (PDDA/GNPs) _n /PDDA was first constructed on the gold electrode by electrostatic layer-by-layer self-assembly of poly(diallyldimethylammonium chloride) (PDDA) and gold nanoparticles (GNPs). Glucose oxidase (GOx) was then sorbed into this BMF by dipping the BMF-modified electrode into a GOx solution. The assembly of the BMF was monitored and tested via UV-vis spectroscopy and cyclic voltammetry (CV). The ferrocenemethanol-mediated cyclic voltammograms obtained from the gold electrode modified with the (PDDA/GNPs) _n /PDDA/GOx indicated that the assembled GOx remained electrocatalytically active for the oxidation of glucose. Analysis of the voltammetric signals showed that the surface coverage of active enzyme was a linear function of the number of PDDA/GNPs bilayers. This result confirmed the penetration of GOx into the BMF and suggests that the BMF-based enzyme film forms in a uniform manner. Electrochemical impedance measurements revealed that the biosensor had a lower electron transfer resistance (R _et) than that of a sensor prepared by layer-by-layer assembly of PDDA and GOx, due to the presence of gold nanoparticles. The sensitivity of the biosensor for the determination of glucose, which could be controlled by adjusting the number of PDDA/GNPs bilayers, was investigated.     

Investigation of Electrosorption of Organic Molecules onto Gold and Nickel Electrodes Using an Electrochemical Quartz Crystal Microbalance

The adsorption behaviors of two organic molecules, benzoic acid (BA) and 2-butyne-1,4-diol (BD), on metal electrodes have been studied using an electrochemical quartz crystal microbalance (EQCM) combined with the cyclic voltammetry technique. In the range of potentials studied, BA molecules were adsorbed onto an electrodeposited gold electrode with a saturation concentration of 5.0 × 10¹⁴ molecules/cm². It was found that the Frumkin isotherm model was most suitable to depict the electrosorption behavior. The isotherm parameters by nonlinear fitting, which agreed with the literature values, implied BA was chemisorbed on the gold surface. For BD on an electrodeposited nickel electrode, the equivalent molar mass of the reaction species was calculated on the basis of the voltammetry curve and mass curve, which were obtained simultaneously during the potential scan. The analysis of EQCM data for the electrosorption of BD on gold and nickel electrodes showed an irreversible characteristic; the latter effectively inhibited the hydrogen evolution reaction.     

基于碳酸钙-金纳米粒子无机杂化复合物固定硫堇的新方法及其在过氧化氢传感器中的应用

通过静电作用,经碳酸钙-金纳米粒子(CaCO3-AuNPs)无机杂化复合物实现了电活性物质硫堇(Th)在金电极表面的有效固定.AuNPs静电吸附到CaCO3微球表面形成CaCO3-AuNPs无机杂化复合物,该杂化复合物具有微孔结构、大的表面积和好的生物相容性,使得Th的固定量和稳定性大大提高.探讨了Th修饰电极在过氧化...     

Achieving Direct Electrical Connection to Glucose Oxidase Using Aligned Single Walled Carbon Nanotube Arrays

Direct electron transfer between an electrode and the redox active centre of glucose oxidase, flavin adenine dinucleotide (FAD), is probed using carbon nanotube modified gold electrodes. Gold electrodes are first modified with a self‐assembled monolayer of cysteamine and then shortened single walled carbon nanotubes (SWNT) are aligned normal to the electrode surface by self‐assembly. The electrochemistry of these aligned nanotube electrode arrays is initially investigated using potassium ferricyanide which showed SWNT act as nanoelectrodes with the ends of the tubes more electrochemically active than the walls. Subsequently the nanotubes are plugged into the enzymes in one of two ways. In the first method, native glucose oxidase is covalently attached to the ends of the aligned tubes which allowed close approach to FAD and direct electron transfer to be observed with a rate constant of 0.3 s^?1. In the second strategy, FAD was attached to the ends of the tubes and the enzyme reconstituted around the surface immobilized FAD. This latter approach allowed more efficient electron transfer to the FAD with a rate constant of 9 s^?1.     

EQCM Study of Iridium Anodic Oxidation in H2SO4 and KOH Solutions

In the present study anodic oxidation of iridium layer formed thermally on a gold‐sputtered quartz crystal electrode has been investigated by electrochemical quartz crystal microgravimetry (EQCM) in the solutions of 0.5 M H₂SO₄ and 0.1 M KOH. The emphasis here has been put on the microgravimetric behavior of iridium as a metal, because a few previous EQCM studies reported in literature have been devoted to iridium oxide films (IROFs). The objective pursued here has been to elucidate the nature of the main voltammetric peaks, which occur at different ranges of potential in the solutions investigated. It has been found that anodic oxidation of iridium electrode in 0.5 M H₂SO₄ and 0.1 M KOH solutions is accompanied by irregular fluctuations of the electrode mass at 0.4 V<E<0.8 V followed by regular increase in mass at 0.8 V<E<1.2 V. The cathodic process initially, at 1.2 V>E>0.9 V, proceeds without any or with slight increase in electrode mass, whereas at E<0.8 V a regular decrease in mass is observed. It has been found that mass to charge ratio characterizing the processes of interest is 2 to 3 g F^?1in acidic medium, whereas in the case of alkaline one it is 4 to 6 g F^?1. The main pair of peaks seen in the voltammograms of Ir electrode in alkaline medium at E<0.8 V is attributable to redox transition Ir(0)→Ir(III), whereas those observed in the case of acidic medium at E>0.8 V should be related to the redox process Ir(0)→Ir(IV) going via intermediate stage of Ir(III) formation. As a consequence of these redox transitions, the gel‐like surface layer consisting of Ir(III) or Ir(IV) hydrous oxides forms on the electrode surface.