Gold Microdisk Electrode Research Articles

Electrochemical Discrimination of Salbutamol from Its Excipients in VentolinTM at Nanoporous Gold Microdisc Arrays.

The emergence of specific drug–device combination products in the inhalable pharmaceutical industry demands more sophistication of device functionality in the form of an embedded sensing platform to increase patient safety and extend patent coverage. Controlling the nebuliser function at a miniaturised, integrated electrochemical sensing platform with rapid response time and supporting novel algorithms could deliver such a technology offering. Development of a nanoporous gold (NPG) electrochemical sensor capable of creating a unique fingerprint signal generated by inhalable pharmaceuticals provided the impetus for our study of the electrooxidation of salbutamol, which is the active bronchodilatory ingredient in VentolinTM formulations. It was demonstrated that, at NPG-modified microdisc electrode arrays, salbutamol is distinguishable from the chloride excipient present at 0.0154 M using linear sweep voltammetry and can be detected amperometrically. In contrast, bare gold microdisc electrode arrays cannot afford such discrimination, as the potential for salbutamol oxidation and chloride adsorption reactions overlap. The discriminative power of NPG originates from the nanoconfinement effect for chloride in the internal pores of NPG, which selectively enhances the electron transfer kinetics of this more sluggish reaction relative to that of the faster, diffusion-controlled salbutamol oxidation. Sensing was performed at a fully integrated three-electrode cell-on-chip using Pt as a quasi-reference electrode.

Direct Measurement of Total Vesicular Catecholamine Content with Electrochemical Microwell Arrays.

We have designed and fabricated a microwell array chip (MWAC) to trap and detect the entire content of individual vesicles after disruption of the vesicular membrane by an applied electrical potential. To understand the mechanism of vesicle impact electrochemical cytometry (VIEC) in microwells, we simulated the rupture of the vesicles and subsequent diffusion of entrapped analytes. Two possibilities were tested: (i) the vesicle opens toward the electrode, and (ii) the vesicle opens away from the electrode. These two possibilities were simulated in the different microwells with varied depth and width. Experimental VIEC measurements of the number of molecules for each vesicle in the MWAC were compared to VIEC on a gold microdisk electrode as a control, and the quantified catecholamines between these two techniques was the same. We observed a prespike foot in a significant number of events (∼20%) and argue this supports the hypothesis that the vesicles rupture toward the electrode surface with a more complex mechanism including the formation of a stable pore intermediate. This study not only confirms that in standard VIEC experiments the whole content of the vesicle is oxidized and quantified at the surface of the microdisk electrode but actively verifies that the adsorbed vesicle on the surface of the electrode forms a pore in the vicinity of the electrode rather than away from it. The fabricated MWAC promotes our ability to quantify the content of vesicles accurately, which is fundamentally important in bioanalysis of the vesicles.

Diffusion-limited biosensing of dissolved oxygen by direct electron transfer-type bioelectrocatalysis of multi-copper oxidases immobilized on porous gold microelectrodes

Diffusion-controlled amperometric biosensors for dissolved oxygen (O2) were constructed by immobilization of multi-copper oxidases (copper efflux oxidase and bilirubin oxidase) on porous gold microdisk electrodes fabricated by anodization in a glucose solution. The immobilized enzymes rapidly consumed O2 near the electrode at potentials more negative than 0.2 V vs. Ag|AgCl|sat. KCl via direct electron transfer-type bioelectrocatalysis and the reduction current reached the steady state limiting value under static conditions. The fabricated biosensor exhibited a linear response to dissolved O2 concentration and was almost identical to the theoretical sensor, based on nonlinear diffusion of O2 around the microdisk electrode. The biosensor response was fast enough to monitor the catalytic consumption of dissolved O2 by glucose oxidase and exhibited storage stability for more than six days.

Electroreduction of 2,4,6-Trinitrotoluene in Room Temperature Ionic Liquids: Evidence of an EC2 Mechanism

The reduction of 2,4,6-trinitrotoluene (TNT) has been studied in eight room temperature ionic liquids (RTILs) on a gold (Au) microdisk electrode and a Au thin film electrode (TFE). Three reduction peaks were observed in all RTILs, corresponding to the reductions of each of the three nitro groups in the TNT structure. TNT was the easiest to reduce in imidazolium RTILs, followed by pyrrolidinium and then tetraalkylphosphonium. Diffusion coefficients (D) and electron counts (n) were calculated from potential-step chronoamperometry on the first reduction peak. D’s ranged from 0.7 × 10–11 to 4.1 × 10–11 m2 s–1, and a plot of D against the inverse of viscosity was linear, indicating that the Stokes–Einstein relation holds well for TNT in RTILs. The electron count was one in most RTILs—in stark contrast to the widely accepted six-electron reduction in protic solvents. An electrogenerated red solid was formed after the first reduction peak, believed to be an azo (or azoxy) compound formed by dimerization of two T...

Application of Asymmetric Marcus–Hush Theory to Voltammetry in Room-Temperature Ionic Liquids

Asymmetric Marcus–Hush (AMH) theory is applied for the first time in ionic solvents to model the voltammetric reduction of oxygen in 1-butyl-1-methylpyrrolidinium bis-(trifluoromethylsulfonyl)-imide and of 2-nitrotoluene (2-NT), nitrocyclopentane (NCP), and 1-nitro-butane (BuN) in trihexyltetradecylphosphonium tris(pentafluoroethyl)trifluorophosphate on a gold microdisc electrode. An asymmetry parameter, γ, was estimated for all systems as −0.4 for the reduction of oxygen and −0.05, 0.25, and 0 ± 0.05 for the reductions of 2-NT, NCP, and BuN, respectively, which suggests equal force constants of reactants and products in the case of 2-NT and BuN and unequal force constants for oxygen and NCP where the force constants of the oxidized species are greater than the reduced species in the case of oxygen and less than the reduced species in the case of NCP. Previously measured values for α, the Butler–Volmer transfer coefficient, reflect this in each case. Where appreciable asymmetry occurs, AMH theory was seen...

One-Electron Reduction of 2-Nitrotoluene, Nitrocyclopentane, and 1-Nitrobutane in Room Temperature Ionic Liquids: A Comparative Study of Butler–Volmer and Symmetric Marcus–Hush Theories Using Microdisk Electrodes

The voltammetry for the reduction of 2-nitrotoluene at a gold microdisk electrode is reported in two ionic liquids: trihexyltetradecylphosphonium tris(pentafluoroethyl)trifluorophosphate ([P14,6,6,6][FAP]) and 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Emim][NTf2]). The reduction of nitrocyclopentane (NCP) and 1-nitrobutane (BuN) was investigated using voltammetry at a gold microdisk electrode in the ionic liquid [P14,6,6,6][FAP]. Simulated voltammograms, generated through the use of Butler–Volmer theory and symmetric Marcus–Hush theory, were compared to experimental data, with both theories parametrizing the data similarly well. An experimental value for the Marcusian parameter, λ, was also determined in all cases. For the reduction of 2-nitrotoluene, this was 0.5 ± 0.1 eV in both solvents, while for NCP and BuN in [P14,6,6,6][FAP], it was 2 ± 0.1 and 5 ± 0.1 eV, respectively. This is attributed to the localization of charge on the nitro group and the primary nitro alkyl’s increased interaction with the environment, resulting in a larger reorganization energy.

Propagation of U(V)-reduction in the presence of U(IV) aggregate in a weakly acidic solution

Reduction processes of UO22+ in weakly acidic solutions were investigated based on electrochemical and spectrophotometric measurements. A reversible one-electron reduction of UO22+ to UO2+ and a further irreversible reduction of UO2+ were observed voltammetrically at a gold microdisk electrode in weakly acidic perchlorate solutions of pH from 2.0 to 3.5. Aggregates of U(IV) were formed as a deposit on the electrode and a colloid in the bulk solution, when the controlled-potential bulk electrolysis was carried out at a gold gauze electrode, even though the potential applied was that available for the first one-electron reduction wave of UO22+ observed at a gold microdisk electrode. It was elucidated that the aggregate was produced by the combination of the one-electron reduction to UO2+ and the disproportionation of UO2+ producing U(IV) and UO22+. The aggregate enhanced the rate of the disproportionation of UO2+, and hence the reduction current of UO22+ increased abruptly when a definite amount of aggregate was formed on the electrode, in the solution, or both. The aggregate of a two-electron reduction product of UO22+ was determined to be poorly crystallized UO2 of fluorite-type structure, and the characteristics of the aggregate were different from the crystalline UO2

Differential linear scan voltammetry: analytical performance in comparison with pulsed voltammetry techniques

We report here on differential linear scan voltammetry, DLSV, that combines the working principles of linear scan voltammetry, LSV, and the numerous existing pulsed voltammetry techniques. DLSV preserves the information from continuous interrogation in voltage and high accuracy that LSV provides about electrochemical processes, and the much better sensitivity of differential pulsed techniques. DLSV also minimizes the background current compared to both LSV and pulsed voltammetry. An early version of DLSV, derivative stationary electrode polarography, DSEP, had been proposed in the 1960s but soon abandoned in favor of the emerging differential pulsed techniques. Relative to DSEP, DLSV takes advantage of the flexibility of discrete smoothing differentiation that was not available to early investigators. Also, DSEP had been explored in pure solutions and with reversible electrochemical reactions. DLSV is tested in this work in more challenging experimental contexts: the measurement of oxygen with a carbon fiber microelectrode in buffer, and with a gold microdisc electrode exposed to a live biological preparation. This work compares the analytical performance of DLSV and square wave voltammetry, the most popular pulsed voltammetry technique.

PH-dependent voltammetric responses of microdisc gold electrodes modified with thiotic acid self-assembled monolayers

This paper describes the pH dependence of cyclic voltammograms (CVs) of hexacyanoferrate(III) on microdisc gold electrodes (25 μm in diameter) modified with thioctic acid (TA) monolayers. The use of a microdisc electrode made it possible to directly compare the experimental CV data with theoretical simulation, and thus to discuss the main factors affecting voltammetric responses in ion-channel sensors (ICSs) based on electrostatic repulsion. Whereas unmodified gold microdisc electrodes gave very similar sigmoidal CVs at pH 2.5–7.5, TA-modified microdisc electrodes exhibited gradual decrease in the reduction current of from pH 2.5 to 5.5 due to the deprotonation of the terminal –COOH groups of TA monolayers. The experimental pH dependence was consistent with theoretical quasi-reversible steady-state voltammograms reflecting the Frumkin effect, suggesting that the changes in the electrode reaction kinetics affected by electrode surface charge would be one of the major factors that offer voltammetric response in such ICSs.

Investigating the Mechanism and Electrode Kinetics of the Oxygen|Superoxide (O2|O2•−) Couple in Various Room-Temperature Ionic Liquids at Gold and Platinum Electrodes in the Temperature Range 298−318 K

The reduction of oxygen was studied over a range of temperatures (298−318 K) in n -hexyltriethylammonium bis(trifluoromethanesulfonyl)imide, [N6,2,2,2][NTf2], and 1-butyl-2,3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C4dmim][NTf2] on both gold and platinum microdisk electrodes, and the mechanism and electrode kinetics of the reaction investigated. Three different models were used to simulate the CVs, based on a simple electron transfer (‘E’), an electron transfer coupled with a reversible homogeneous chemical step (‘ECrev’) and an electron transfer followed by adsorption of the reduction product (‘EC(ads)’), and where appropriate, best fit parameters deduced, including the heterogeneous rate constant, formal electrode potential, transfer coefficient, and homogeneous rate constants for the ECrev mechanism, and adsorption/desorption rate constants for the EC(ads) mechanism. It was concluded from the good simulation fits on gold that a simple E process operates for the reduction of oxygen in [N6...

SAM-modified microdisc electrode arrays (MDEAs) with functionalized carbon nanotubes

The construction of a novel voltammetric transducer employing mixed acid-chopped, oxidatively modified, single-walled carbon nanotubes (HOOC–SWNT–COOH) covalently coupled to alkane thiol self-assembled monolayers (SAMs) that were deposited onto gold microdisc electrode arrays (MDEA-Au) (1296 microdiscs, 100 μm diameter each) has been demonstrated. The chemisorption of cysteamine (CA) and 11-amino-1-undecanethiol (11-AUT) SAMs onto the MDEAs resulted in MDEA-Au|CA and MDEA-Au|11-AUT, which when conjugated with the HOOC–SWNT–COOH produced MDEA-Au|CA|SWNT and MDEA-Au|11-AUT|SWNT electrodes. Electrodes were characterized by multiple scan rate cyclic voltammetry in the presence of ferrocene monocarboxylic acid and by AFM and compared. As expected, CA modification of MDEA-Au produced a disordered film that did not alter the effective surface area of the MDEA while 11-AUT modification resulted in passivation and a 50% reduction in effective surface area. The conjugation of the SWNT to the CA-modified and the 11-AUT-modified MDEA-Au surface returned well-resolved voltammograms with highest peak currents ( I pa = 5.54 × 10 −4 A, I pc = 2.34 × 10 −4 A at 100 mV/s). The effective area of MDEA-Au|CA|SWNT was increased by 200% while that of MDEA-Au|11-AUT|SWNT was increased by 100%. AFM showed SWNTs to be vertically oriented on the CA-modified surface but to be horizontally arranged and bundled on the 11-AUT surface.

Electrochemical Sensor Array for Glucose Monitoring Fabricated by Rapid Immobilization of Active Glucose Oxidase within Photochemically Polymerized Hydrogels

Currently, monitoring blood glucose levels for diabetic patients is invasive and painful, involving pricking the finger to obtain a blood sample three to four times daily. The need for frequent tests and pain involved with testing leads to poor compliance. In order to raise compliance, we propose to create an implantable electrochemical sensor array that would monitor glucose levels continuously. Glucose sensor arrays were fabricated on gold electrodes on flexible polyimide sheets by photopolymerization of the biocompatible polymer poly(ethylene glycol) diacrylate (PEG-DA) to develop hydrogels and encapsulate the sensing elements. Using conventional silicon fabrication methods, arrays of five gold microdisk electrodes were fabricated using lift-off photolithography and sputtering techniques. A redox polymer was then attached electrostatically to the electrode, and glucose oxidase was entrapped inside the hydrogel on the array of electrodes by ultraviolet-initiated photopolymerization of PEG-DA. When the array of fabricated sensors was sampled together the elements behaved like one large electrode with peak current equivalent to the sum of individual array elements. The enzyme, glucose oxidase, catalyzed the oxidation of glucose and then exchanged electrons with the redox polymer in the hydrogel. The entrapped glucose oxidase was found to respond linearly to increasing glucose concentrations (0-360 mg/dl), as determined using cyclic voltammetry. The fabricated microarray sensors were individually addressable and showed no cross talk between adjacent array elements as assessed using cyclic voltammetry. We have fabricated an array of glucose sensors on flexible polyimide sheets that exhibits the desired linear response in the biological range.

Continuous measurement of acute toxicity in water using a solid state microrespirometer

This work presents the first true solid-state microrespirometer. It consists on a naturally developed biofilm of Pseudomonas aeruginosa over a Nafion modified array of gold microdisc electrodes. Such a device can be used to monitor acute toxicity in water streams based on the oxygen reduction current recorded at the microelectrode array. When a healthy biofilm is exposed to a toxic stream the bacteria conforming stop breathing which results in an immediate increase of the oxygen supply reaching the microelectrode array. The fabrication and use of such a microrespirometer is described, and a possible operation mode for a real application is presented. Given that different waters may contain toxic compounds of diverse nature, the respirometer presented here provides qualitative information only.

Anodic stripping voltammetry at in situ bismuth-plated carbon and gold microdisc electrodes in variable electrolyte content unstirred solutions

Carbon and gold microdisc electrodes (30 and 10 μm, respectively) have been tested as substrates for in situ bismuth film plating from unstirred solutions of variable acetate buffer content and were subsequently used in the anodic stripping voltammetry determination of Pb(II) and Cd(II) ions. The effects of Bi(III) concentration, analyte accumulation time, stirring as well as supporting electrolyte content have been studied. Under optimal conditions good voltammetric responses were obtained by means of square wave anodic stripping voltammetry in unstirred analyte solutions of 5 × 10 −8 to 10 −6 M, even in the absence of added buffer. In an indicative application, Pb(II) ion levels were determined in tap water using bismuth-plated carbon microdisc electrodes.

Kinetic Analysis of the Reaction between Electrogenerated Superoxide and Carbon Dioxide in the Room Temperature Ionic Liquids 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide and Hexyltriethylammonium Bis(trifluoromethylsulfonyl)imide

The reduction of oxygen in the presence of carbon dioxide has been investigated by cyclic voltammetry at a gold microdisk electrode in the two room-temperature ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][N(Tf)2]) and hexyltriethylammonium bis(trifluoromethylsulfonyl)imide ([N6222][N(Tf)2]). With increasing levels of CO2, cyclic voltammetry shows an increase in the reductive wave and diminishing of the oxidative wave, indicating that the generated superoxide readily reacts with carbon dioxide. The kinetics of this reaction are investigated in both ionic liquids. The reaction was found to proceed via a DISP1 type mechanism in [EMIM][N(Tf)2], with an overall second-order rate constant of 1.4 ± 0.4 × 103 M-1 s-1. An ECE or DISP1 mechanism was determined to be the most likely pathway for the reaction in [N6222][N(Tf)2], with an overall second-order rate constant of 1.72 ± 0.45 × 103 M-1 s-1.

Integrated Microdisk Gold Electrode Modified with Metal-porphyrin and Metal-phthalocyanines for Nitric Oxide Determination in Biological Media

An integrated gold microdisk electrode was constructed and modified with metal-porphyrin or metal-phthalocyanines for NO determination in biological media. Microanalysis of NO using square wave anodic stripping voltammetry in 1 x 10 - 2 M HClO 4 was optimal when the accumulation potential was 0.1 V, frequency 100 Hz, and the scan rate was 200 mV/s. When the electrode was modified with metal-porphyrin or metal-phthalocyanines, the anodic peak currents of NO increased due to the catalytic oxidation of NO. In case of Fe(II)-phthalocyanine modified electrode, the peak currents remarkably increased and the sensitivity was high. The calibration curve had good linearity in the range from 3.6 x 10 - 5 M to 7.2 x 10 - 7 M, and the detection limit was 5.7 x 10 - 7 M. For the structural stability and increased sensitivity, Fe(II)-phthalocyanine modified gold microdisk electrode coated with Nafion was applied to determination of NO released from cultured macrophase.

Voltammetry of Oxygen in the Room-Temperature Ionic Liquids 1-Ethyl-3-methylimidazolium Bis((trifluoromethyl)sulfonyl)imide and Hexyltriethylammonium Bis((trifluoromethyl)sulfonyl)imide: One-Electron Reduction To Form Superoxide. Steady-State and Transient Behavior in the Same Cyclic Voltammogram Resulting from Widely Different Diffusion Coefficients of Oxygen and Superoxide

The electrochemical reduction of oxygen in two different room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([EMIM][N(Tf)2]) and hexyltriethylammonium bis((trifluoromethyl)sulfonyl)imide ([N6222][N(Tf)2]) was investigated by cyclic voltammetry at a gold microdisk electrode. Chronoamperometric measurements were made to determine the diffusion coefficient, D, and concentration, c, of the electroactive oxygen dissolved in the ionic liquid by fitting experimental transients to the Aoki model. [Aoki, K.; et al. J. Electroanal. Chem. 1981, 122, 19]. A theory and simulation designed for cyclic voltammetry at microdisk electrodes was then employed to determine the diffusion coefficient of the electrogenerated superoxide species, O2•-, as well as compute theoretical voltammograms to confirm the values of D and c for neutral oxygen obtained from the transients. As expected, the diffusion coefficient of the superoxide species was found to be smaller than that of the oxygen in both ionic liquids. The diffusion coefficients of O2 and O2•- in [N6222][N(Tf)2], however, differ by more than a factor of 30 (DO2 = 1.48 × 10-10 m2 s-1, DO2•− = 4.66 × 10-12 m2 s-1), whereas they fall within the same order of magnitude in [EMIM][N(Tf)2] (DO2 = 7.3 × 10-10 m2 s-1, DO2•− = 2.7 × 10-10 m2 s-1). This difference in [N6222][N(Tf)2] causes pronounced asymmetry in the concentration distributions of oxygen and superoxide, resulting in significant differences in the heights of the forward and back peaks in the cyclic voltammograms for the reduction of oxygen. This observation is most likely a result of the higher viscosity of [N6222][N(Tf)2] in comparison to [EMIM][N(Tf)2], due to the structural differences in cationic component.

The Electrochemical Oxidation of N,N‐Diethyl‐p‐Phenylenediamine in DMF and Analytical Applications. Part I: Mechanistic Study

AbstractThe electrochemical oxidation of N,N‐diethyl‐p‐phenylenediamine in dimethylformamide has been studied at platinum and gold microdisk electrodes of various radii between 6.7 and 66 μm. The voltammetric responses revealed two electrochemically reversible waves the second of which becomes larger at higher concentrations and bigger electrode radii. The voltammetric signals have been modelled and the electrochemical oxidation reaction is not inconsistent with an ECrevECE reaction. Kinetic parameters are reported.

Studies of the electrochemical reduction of 4-nitrophenol in dimethylformamide: evidence for a change in mechanism with temperature

The electrochemical reductions of 4-nitrophenol, 2-cyanophenol and 4-cyanophenol were studied in dimethylformamide (DMF) at gold microdisk electrodes at elevated temperatures up to 110°C. It was inferred that there is a change in mechanism for the reduction of 4-nitrophenol near 25°C. Over the entire temperature range 4-nitrophenol is reduced to the corresponding radical anion, with subsequent rapid protonation by the parent molecule to form a radical species. At temperatures above 25°C it appears that this species can undergo further electrochemical reduction at the gold electrode, introducing a new voltammetric reduction wave, contrasting the response observed at platinum.

Study of electrochemical properties of pyrrolidinofullerenes by microelectrode voltammetry

The electrochemical behavior of nine pyrrolidinofullerenes has been investigated by cyclic voltammetry on a gold microdisk electrode. Four reversible reduction peaks and two irreversible reduction peaks are observed for each fullerene derivative. The half-wave potentials of all pyrrolidinofullerenes are more negative than those of C 60 itself. The diffusion coefficient of these compounds is measured by their steady-state voltammograms.