Comparison of Pt and Ni foil electrodes for amperometric sensing of ascorbic acid

Metal foils made of Pt and Ni show amperometric responses to ascorbic acid (H₂A). This study compares the sensitivity of a Pt foil electrode to that of a Ni foil electrode. Both electrodes were highly sensitive to H₂A; the Pt foil electrode was more sensitive at an applied potential of 0.2 V vs. Ag/AgCl in KOH than it was in H₂SO₄ and Na₂SO₄. These electrodes showed excellent selectivity for H₂A over glucose, fructose, sucrose, tartaric acid, citric acid, oxalic acid, and sodium benzoate. Both electrodes displayed rapid responses and good linearity levels in the range from 0.57 to 5.68 mM. Compared to the Ni foil electrode, the Pt foil electrode possessed better long-term stability, because it maintained 99% of its initial sensitivity for more than 67 days.     

Synthetic semiconductor diamond electrodes: kinetics of some redox reactions

Linear-sweep voltammograms were recorded on boron-doped diamond thin-film electrodes in K₃Fe(CN)₆, K₄Fe(CN)₆, quinone, and hydroquinone solutions; transfer coefficients and rate constants for anodic and cathodic reactions were determined. The reaction rates were compared to the diamond samples' electrical characteristics measured by the impedance spectroscopy techniques. The polarization (faradaic) resistance of a redox reaction at its equilibrium potential was shown to be proportional to the specific resistance of diamond samples. Reaction rate constants for the quinone/hydroquinone system appeared to be significantly less than those for the Fe(CN)₆^3?/4? system.     

Studies of CO tolerance on modified gas diffusion electrodes containing ruthenium dispersed on carbon

Studies of the CO tolerance for the hydrogen oxidation reaction (hor) on a polymer electrolyte fuel cell were conducted on modified gas diffusion electrodes with CO filtering layers formed by ruthenium dispersed onto carbon (Ru/C) and having platinum dispersed on carbon (Pt/C) in the catalyst layer. Different configurations for gas diffusion electrodes containing Ru hydrated-oxide (RuO_xH_y/C) layers were tested in order to understand the CO oxidation mechanism on these catalysts. Single cell polarization results indicated that a direct contact between Ru distributed on the diffusion layer and the catalyst layer is required to achieve CO tolerance. Results also show that an enhanced activity is achieved for the hor in the presence of CO with PtRu/C in the catalyst layer obtained by an impregnation method. Also, an electrode formed by a RuO_xH_y/C filtering layer in contact with a catalyst layer with PtRu/C showed the highest CO-tolerance. For this system, the cell voltage loss was only 65 mV at 1 A cm^?2 with H₂ + 100 ppm CO, with respect to the value obtained with pure hydrogen.     

Simultaneous determination of phenolic compounds by using a dual enzyme electrodes system

Simultaneous amperometric determination of two phenolic compounds was performed by using a tyrosinase (TYR) electrode and a TYR-peroxidase (POD) bienzyme electrode on the basis of differences in the selectivities for two phenolic compounds between the two enzyme electrodes. Catechol and p-cresol contained in the same solution were determined simultaneously with the present system. The response of the bienzyme electrode was greater than the sum of the response of the TYR electrode and that of a POD electrode. This amplification effect may be responsible for the differences in the selectivities. Amplification mechanisms were also discussed.     

Anodic oxidation of free nitric oxide at gold electrodes modified by a film of trans-[Ru(III)(NH3)4(SO4)4pic]+ and molybdenum oxide

Gold surfaces were modified with an electrochemically deposited layer of non-stoichiometric molybdenum oxides. At these surfaces, trans-[Ru(III)(NH₃)₄(SO₄)4pic]⁺ complex was incorporated in a controlled way by cycling the potential consecutively in the range +0.50 to ?0.25 V at pH 2.6. Very reproducible voltammetric curves corresponding to the electrochemical process of the ruthenium complex were obtained, confirming the immobilisation of the material into the molybdenum oxide film. The anodic oxidation of nitric oxide (NO) at pH 7.4 was investigated at the modified electrode containing the molybdenum oxide+trans-[Ru(III)(NH₃)₄(SO₄)4pic]⁺ complex and an enhancement in the current response was observed compared with the signal at a bare electrode. The rate for NO electrochemical oxidation was dependent on the amount of catalytic ruthenium sites dispersed into the molybdenum oxide film. A linear relationship between current signals measured by square wave voltammetry and NO concentration was obtained in the 0–10 μM range. The applicability of the modified electrode as a sensor for real-time NO monitoring was also demonstrated.     

Electrosynthesis of a new enzyme-modified electrode for the amperometric detection of glucose

The electrooxidation of a new aminonaphthalene compound modified with a carboxylic acid function, 1-(5-aminonaphthyl)ethanoic acid (ANEA) was studied, with a view to binding glucose oxidase (GOD) covalently. Its electrooxidation leads to thin films (PANEA) on bare glassy carbon (GC) platinum (Pt) electrodes and also on prepolymerized aminonaphthoquinone (PANQ) films. Polymer-modified bilayer electrodes Pt/PANQ/PANEA were then constructed. GOD was covalently bound onto these electrodes, leading to a Pt/PANQ/PANEA/GOD glucose sensor. The amperometric response to glucose was monitored in aerated and deaerated conditions. The results show very efficient covalent binding of GOD and good enzyme activity on the electrode surface.     

The development of new microelectrode gas sensors: an odyssey: Part IV. O2, CO2 and N2O reduction at unshielded gold microdisc electrodes

In this work, we describe the reduction of O₂, CO₂ and N₂O, in the presence of each other, at an unshielded gold microdisc electrode, in dimethylsulfoxide (DMSO) as the solvent and with a Ag quasi-reference electrode. Carbon dioxide and nitrous oxide, when present on their own in N₂, have been shown previously to be reduced at very similar potentials. So, it was not necessarily anticipated that gas mixtures containing both these two gases could be determined analytically. However, remarkably, a well resolved new CO₂ wave, which was ideal for analytical purposes, was observed under our experimental conditions in the presence of O₂ and N₂O. Furthermore, and most importantly, no cross-interference from the O₂, N₂O, and the new CO₂ reactions, was evident when the N₂O concentration was in excess of that of CO₂ (the normal anaesthetic expired gas and blood gas analysis situation). The O₂, CO₂ and N₂O concentrations were determined from the limiting currents of the well-separated waves.     

Impedance of ruthenium electrodes in sulphuric acid solution

Electrochemical impedance spectroscopy was employed to characterize three different types of Ru electrodes, anodically polarized and measured at +0.85 V versus SCE in 0.5 mol dm^?3 H₂SO₄, both in their non-electrochemically treated (NET states), and also following various potential/time treatment programmes (ET states). On the basis of experimental data and results of the complex non-linear curve fitting procedure, the impedance/frequency response is for all three Ru electrodes in either the NET or ET state, modelled generally as two impedance branches in parallel. The first, an almost pure capacitative impedance is ascribed to the fast and reversible charging/discharging of the oxide film surface, proceeding through oxidation/reduction of electron sites coupled by the surface deprotonation/protonation. The second impedance consists of the resistive impedance and two transmission line elements in the series connection. This impedance is ascribed to the oxide film equilibration process which proceeds mainly through diffusion-like transport of reaction species within a heterogeneous structure of the oxide film. In dependence on the type of Ru electrode and particularly on the electrode state (NET or ET) there are significant differences in contribution of each impedance to the overall impedance/frequency response. Thus predominance of diffusion-like impedance at medium and low frequencies is observed for all three Ru electrodes in their NET state. This indicates the contribution of the oxide film equilibration process in the overall electrode response. If, however, Ru electrodes are treated by defined potential/time treatment programmes, the electrode impedance/frequency response of each electrode is changed continuously, showing increase of the extent of the surface reaction and facilitated and accelerated oxide film equilibration. For some types of Ru electrodes, pure and low capacitative impedance behaviour in almost the whole frequency region is observed after only few potential steps applied.     

Ruthenium purple-containing zeolite modified electrodes and their application for the detection of glucose

Zeolite-entrapped ruthenium purple particles (denoted NaY ∣ RP) were prepared by exchanging Fe³⁺ into zeolite Y (NaY) and reacting further with ruthenium hexacyanide (Ru(CN)₆^4?). Although XRD and diffuse-reflectance UV-vis absorption analyses suggested that the RP_(NaY) particles were basically amorphous, the resulting electrode prepared from these particles and glucose oxidase (GOx) displayed a remarkable sensitivity to glucose. The limiting current showed a linear relationship with the bulk activity of glucose up to 4 mM (pH 5.1, phosphate), covering a range from 10^?6 to 10^?3 M. Flow-injection analysis, in addition, showed the detection limit reaching a level as low as 0.1 μM. Data simulation showed that the electrode sensitivity followed an electrocatalytic (EC′) mechanism based on the reduction of H₂O₂ by the reduced ruthenium purple. Accordingly, the reaction rate constant (pseudo-first-order) and the effective activity of RP were determined to be 110 M^?1 s^?1 (pH 5) and 1.5×10^?9 mol cm^?2, respectively. Electrochemical impedance spectroscopic (EIS) analysis showed that the charge-transfer resistance of the zeolite-electrode decreased systematically with the stepwise addition of glucose into the system. The exchange rate constant (k_o) and the diffusion coefficient of electrons (D_e) in the zeolite film were estimated to be 5×10^?6 cm s^?1 and 6×10^?10 cm² s^?1, respectively. These experimental results suggested that the RP sites in the NaY ∣ RP particles were separated widely with the average distance between the adjacent RP sites estimated to be about 1×10^?4 cm or equivalent to 500 supercages, which agreed well with the results obtained from data simulation.     

Amperometric determination of optically active 1-phenylethanol using chiral nitroxyl radical-modified polypyrrole films prepared by electrochemical polymerization

Polymeric chiral nitroxyl radical catalysts were prepared on an electrode surface by electrochemical polymerization of (6S, 7R, 10R)-2,2,7-trimethyl-10-isopropyl-1-azaspiro[5.5]undecane-1-yloxyl precursors containing a pyrrole side chain. This modified electrode gave a reversible electron transfer for the nitroxyl radical/oxoammonium ion redox couple in cyclic voltammetry at +0.72 V vs. Ag|AgCl. The modified electrode was used for electrocatalytic oxidation of (R)-(+)- and (S)-(?)-1-phenylethanol as an optically active sec-alcohol. Cyclic voltammetric studies showed that the catalytic current for the oxidation of (S)-(?)-1-phenylethanol is greatly enhanced as compared with a small enhancement in the oxidation current for the (R)-isomer. The (S)-isomer can be detected selectively in a mixture of (R)-(+)- and (S)-(?)-1-phenylethanol, even in the presence of an excess amount of (R)-isomer.