Modified electrode approaches for nitric oxide sensing

Three different methods for the determination of nitric oxide (NO) in solution are described. These are based, respectively, on the use of a horseradish peroxidase (HRP) biosensor or on electrodes modified with films of redox-active transition metal complexes. In the case of the biosensor the enzyme was electrochemically immobilized onto a glassy carbon (GC) electrode. The activity of HRP is inhibited in presence of NO. Thus, the decrease in activity is correlated to the concentration of NO present in solution. The biosensor responds linearly over the range of 2.7×10⁻⁶-1.1×10⁻⁵ M NO with a detection limit (5% inhibition) of 2.0×10⁻⁶ M. In the case of chemically modified electrodes, particular emphasis is placed on materials capable of catalyzing the oxidation of NO. In terms of electrocatalyst, the discussion will centre on electrodeposited films of 6,17-diferrocenyldibenzo[b,i]5,9,14,18-tetraaza[14]annulen]-nickel(II) and indium(III) hexacyanoferrate(III). The resulting sensors exhibited potent and persistent electroacatalytic activity towards the oxidation of NO with low detection limits (1 μM) and good linear relationship between the catalytic current and NO concentrations. In addition, interference due to the presence of nitrate and nitrite have been significantly reduced. According to these results, the described modified electrodes have been used as sensors for the determination of NO generated by decomposition of a typical NO-donor, such as S-nitroso-N-acetyl-d,l-penicillamine (SNAP). A critical comparison of the various methodologies employed is made.     

Electrocatalytic Oxidation and Flow-Injection Determination of Sulfur-Containing Amino Acids at Graphite Electrodes Modified with a Ruthenium Hexacyanoferrate Film

The electrochemical behavior of sulfur-containing amino acids (cysteine, cystine, and methionine) at graphite electrodes modified with a ruthenium(III) hexacyanoferrate(II) film was studied. Glassy carbon and carbon paste were used as graphite materials. The electrocatalytic oxidation of amino acids at a modified electrode resulted in a decrease in the oxidation potentials of amino acids and an increase in the currents of their oxidation peaks as compared to those observed at an unmodified electrode. The voltammetric characteristics and hydrodynamic conditions for detecting the maximum catalytic current were found. A procedure is proposed for the electrocatalytic determination of cysteine, cystine, and methionine at a carbon-paste electrode modified with an inorganic film of ruthenium(III) hexacyanoferrate(II) under the conditions of flow-injection analysis.     

Electrocatalytic oxidation of hydrazine at cobalt hexacyanoferrate- modified glassy carbon, Pt and Au electrodes

The electrocatalytic oxidation of hydrazine has been studied on glassy carbon, Pt and Au electrodes modified by cobalt hexacyanoferrate (CoHCF) using cyclic voltammetry and rotating disc techniques. It has been shown that the oxidation of hydrazine to nitrogen occurs at the potential coinciding with that of Co(II) to Co(III) transformation in a CoHCF film, where no oxidation signal is observed at a bare glassy carbon electrode. A Tafel plot, derived from RDE voltammograms, exhibits a slope of 150 mV, indicating a one-electron charge transfer process to be the rate-limiting step. The electrocatalytic efficiency of the modified electrode towards hydrazine oxidation depends on solution pH, and the optimum range was found to be located between pH 5 and pH 7. The kinetic behaviour and location of the electrocatalytic process were examined using the W.J. Albery diagnosis table, and it was concluded that the reaction has either a “surface” or a “layer” reaction mechanism. Pt- and Au-CoHCF-modified electrodes show no significant electrocatalytic activity towards hydrazine oxidation. Received: 25 April 1997 / Accepted: 12 August 1997     

Potentiometric determination of potassium cations using a nickel(II) hexacyanoferrate-modified electrode

Electroactive nickel(II) hexacyanoferrate (NiHCF) thin film modified electrodes are effective potentiometric sensors for the determination of potassium ions. The NiHCF films are deposited onto glassy carbon electrodes by repetitive potential cycling in K(3)Fe(CN)(6)/NaNO(3)/Ni(NO(3))(2) solution. The modified electrodes exhibit a linear response to potassium ions in the concentration range 1x10(-3) to 2.0 mol dm(-3), with a near-Nernstian slope (45-49 mV per decade) at 25 degrees C. In the determination of potassium ion in syrups used for treatment of potassium deficiency, the NiHCF-modified electrode gave comparable results to those obtained using flame emission spectrophotometry.     

Hybrid Materials from Carbon Nanotubes,Nickel Tetrasulfonated Phthalocyanine and Thin Polymer Layers for the Selective Electrochemical Activation of Nitric Oxide in Solution

The elaboration of hybrid materials from single‐wall carbon nanotubes (SWCNT) and tetrasulfonated nickel phthalocyanine (NiTSPc) was electrochemically performed to obtain chemically modified electrodes with improved electrocatalytic activity towards the electrooxidation of nitric oxide (NO). The characterization of these hybrid electrodes was achieved by cyclic voltammetry and AFM. The electrochemical performances of the electrodes towards the oxidation of NO were then analyzed by chronoamperometry and the obtained results show that the presence of SWCNT greatly enhances the electrocatalytic performances in terms of current intensity. Additional coatings were then electrodeposited over the hybrid electrodes to act as selective membranes against four major interfering analytes: nitrite, ascorbate, hydrogen peroxide and L ‐arginine. Several polymer coatings were tested to achieve the best balance between sensitivity to NO and selectivity against interferents.     

Electrocatalytic Oxidation and Determination of Sulfite with a Novel Copper‐Cobalt Hexacyanoferrate Modified Carbon Paste Electrode

The electrocatalytic oxidation of sulfite has been studied at a stable electroactive thin film of copper‐cobalt hexacyanoferrate (CuCoHCF) hybrid electrodeposited on a carbon paste electrode (ECMCPE). A linear range of 5 μM to 5 mM of sulfite, with an experimental detection limit of 1 μM, was obtained using the cyclic voltammetric method. The oxidation of sulfite showed no significant fouling effect on the modified electrode surface at sulfite concentrations below 5 mM. The proposed modified electrode exhibited several attractive features, including simple preparation, fast response, good stability and repeatability, and could be applied to sulfite determination in real samples.     

Dopamine Detection Using the Selective and Spontaneous Formation of Electrocatalytic Poly(dopamine) Films on Indium?Tin Oxide Electrodes

In this study, we report the simple and sensitive electrochemical detection of dopamine in the presence of excess ascorbic acid. The detection is based on the spontaneous formation of electrocatalytic poly(dopamine) films on bare indium? tin oxide (ITO) electrodes. The poly(dopamine) films are formed by immersing ITO electrodes in a solution of dopamine and ascorbic acid for 10 min. Afterwards, the electrocatalytic oxidation of hydrazine is measured using modified electrodes. The electrooxidation current of hydrazine increases with increasing dopamine concentration. This method allows a detection limit of 1 nM for dopamine in the presence of 100 µM of ascorbic acid.     

Preparation,Characterization, and Electrocatalytic Properties of Cobalt Oxide and Cobalt Hexacyanoferrate Hybrid Films

Polynuclear mixed‐valent films of cobalt oxide and cobalt hexacyanoferrate (CoOCoHCF) have been deposited on electrode surfaces from a solution of Co²⁺ and Fe(CN)₆^3? ions by repetitive potential cycling method. Simultaneous cyclic voltammetry and electrochemical quartz crystal microbalance measurements demonstrate the steady growth of modified film. The effect of type of monovalent cations as well as acidity of the supporting electrolyte on film growth and redox behavior of resulting film was investigated. In pure supporting electrolyte, electrochemical responses of modified electrode resemble with that of a surface immobilized redox couple. The hybrid film electrodes showed electrocatalytic activity toward oxidation of NADH, hydrazine and hydroxylamine. The feasibility of using our modified electrodes for analytical application was also explored.     

六氰合铁酸铜钴-多壁碳纳米管修饰电极研究

采用电沉积方法制备六氰合铁酸铜钴-多壁碳纳米管复合修饰电极(CuCoHCF-MWCNTs/GCE).研究碳纳米管用量、电解液组成对该修饰电极性能的影响.结果表明,与单一的六氰合铁酸铜钴薄膜修饰电极相比,六氰合铁酸铜钴-多壁碳纳米管复合修饰电极具有更优良的电化学特性,以其催化氧化过氧化氢,峰电流与过氧化氢浓度在3.16×10-5～2.92×10-3mol·L-1范围内呈良好的线性关系,线性回归方程为ip(μA)=0.5529+1.1299C(×10-4mol·L-1),相关系数r=0.9966,检出限为1.75×10-5mol·L-1.     

Influence of Metal Nanoparticles on the Electrocatalytic Oxidation of Glucose by Poly(NiIIteta) Modified Electrodes

Conductive polymeric [Ni^II(teta)]²⁺ (teta=C‐meso‐5,5,7,12,12,14‐hexamethyl‐1,4,8,11‐tetra‐azacyclotetradecane) films (poly(Ni)) have been deposited on the surface of glassy carbon (GC), Nafion (Nf) modified GC (GC/Nf) and Nf stabilized Ag and Au nanoparticles (NPs) modified GC (GC/Ag‐Nf and GC/Au‐Nf) electrodes. The cyclic voltammogram of the resulting electrodes, show a well defined redox peak due to oxidation and reduction of poly(Ni) system in 0.1 M NaOH. They show electrocatalytic activity towards the oxidation of glucose. AFM studies reveal the formation of poly(Ni) film on the modified electrodes. Presence of metal NPs increases electron transfer rate and electrocatalytic oxidation current by improving the communication within the Nf and poly(Ni) films. In the presence of metal NPs, 4 fold increase in current for glucose oxidation was observed.     

Electrocatalytic oxidation and sensitive detection of deferoxamine on nanoparticles of Fe2O3@NaCo[Fe(CN)6]-modified paste electrode

Electrocatalytic oxidation of deferoxamine was studied on carbon paste electrodes modified with nanoparticles of iron(III) oxide core-cobalt hexacyanoferrate-shell using cyclic voltammetry and chronoamperometry. Voltammetric curves represented two quasi-reversible redox transitions which in the presence of deferoxamine, the two anodic peak currents increased, followed by decreases in the corresponding cathodic currents. This indicated that deferoxamine was oxidized on the immobilized cobalt hexacyanoferrate via two electrocatalytic steps. The rate constants, the electron transfer coefficients and the diffusion coefficient involved in the electrocatalytic oxidation of deferoxamine were reported. A sensitive and time-saving determination procedure was developed for the analysis of the drug in both batch and flow systems, and the corresponding analytical parameters were reported. The proposed amperometric method was also successfully applied to the direct assays of spiked human serum with iron–drug complex.     

Fabrication and characterization of copper nanoparticle thin-films and the electrocatalytic behavior

Copper nanoparticles, stabilized by cysteine, have been formed thin-films on indium tin oxide (ITO) electrodes with (3-mercaptopropyl) trimethoxysilane (MPTMS). Scanning electron micrograph (SEM) demonstrates that the films are uniform and the copper nanoparticles are homogeneously distributed on the ITO electrode surface. Electrochemical experiments reveal that the obtained copper nanoparticles modified electrodes show improved electron-transfer characteristics and exhibit high electrocatalytic activity to the reduction of nitrite and nitric oxide, which can be utilized as an efficient electrochemical sensor.     

Cyclic Voltammetry and the Impedance of Electrodes Modified by Indium(III) Hexacyanoferrate Films

The results of studying indium(III) hexacyanoferrate films by the cyclic voltammetry and faradaic impedance methods are presented. It is shown that the synthesis from solutions with different contents of potassium salts leads to the appearance of films with different electrochemical properties. The impedance spectra exhibit semicircles corresponding to the Randles circuit and portions of the Warburg dependence. Parameters of the equivalent circuit corresponding to the obtained spectra are determined.     

Electrocatalysis of Water Oxidation by H2O‐Capped Iridium‐Oxide Nanoparticles Electrodeposited on Spectroscopic Graphite

Electrocatalysis of water oxidation by 1.54 nm IrO_x nanoparticles (NPs) immobilized on spectroscopic graphite electrodes was demonstrated to proceed with a higher efficiency than on all other, hitherto reported, electrode supports. IrO_x NPs were electrodeposited on the graphite surface, and their electrocatalytic activity for water oxidation was correlated with the surface concentrations of different redox states of IrO_x as a function of the deposition time and potential. Under optimal conditions, the overpotential of the reaction was reduced to 0.21 V and the electrocatalytic current density was 43 mA cm^?2 at 1 V versus Ag/AgCl (3 M KCl) and pH 7. These results beneficially compete with previously reported electrocatalytic oxidations of water by IrO_x NPs electrodeposited onto glassy carbon and indium tin oxide electrodes and provide the basis for the further development of efficient IrO_x NP‐based electrocatalysts immobilized on high‐surface‐area carbon electrode materials.     

Determination of ascorbic acid using an electrode modified with cysteine self-assembled gold-platinum nanoparticles

We report on a new type of indium tin oxide (ITO) electrode for sensing ascorbic acid (AA). The ITO film was modified with gold-platinum alloy nanoparticles (Au-Pt NPs) functionalized with a self-assembled film of L-cysteine. The Au-Pt NPs were electrodeposited on the ITO film and characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction. A cyclic voltammetric study revealed that the electrode exhibits excellent electrocatalytic activity towards the oxidation of AA. The calibration plot for AA is linear over the concentration range from 2 to 400???M with a correlation coefficient of 0.9991. The detection limit of AA is 1???M.

Copper oxide-modified glassy carbon electrode prepared through copper hexacyanoferrate–G5-PAMAM dendrimer templates as electrocatalyst for carbohydrate and alcohol oxidation

We describe the modification of a glassy carbon electrode by electro-deposition of copper hexacyanoferrate (CuHCF) in the presence of an amine-terminated dendrimer (PAMAM) as a template. The electrode containing the CuHCF template was cycled in alkaline solution to generate a layer of cupric oxide (CuO). The mechanism of the formation of CuO and its electrocatalytic activity were investigated by cyclic voltammetry. Scanning electron microscopy reveals that the CuO prepared by this method has a meso-porous grid-like appearance. The formation of CuO was identified by XPS analysis of the modified electrode. The ability of the CuHCF film towards the electrocatalytic oxidation of carbohydrates and alcohols was detected using cyclic voltammetry. The over-potential required for carbohydrate and alcohol oxidation is lowered by ~400 mV compared to other chemically modified electrodes reported in the literature. Simple methodology has been adopted in this work for the preparation of the catalytically active electrode, and this work also explains the structure directing effect of dendrimer and its influence on the electrocatalytic oxidation of analytes.     

Detection of Nitrite Using Electrodes Modified with an Electrodeposited Ruthenium-Containing Polymer

Abstract

The electrochemical detection of nitrite in flowing solutions using a polymer modified electrode with electrocatalytic properties is described. The modifier was an electrodeposited polymer film of the [Ru(bpy)₂(vpy)₂+²⁺ complex, where bpy is 2,2′ bypyridyl and vpy is 4-vinylpyridine. The modified electrodes showed the ability to enhance the nitrite response and avoid surface fouling. The electrodeposited films exhibited higher mechanical stability than similar chemisorbed coatings.     

Glassy carbon electrodes modified by multiwalled carbon nanotubes and poly(neutral red): A comparative study of different brands and application to electrocatalytic ascorbate determination

The electrochemical behaviour of glassy carbon electrodes coated with multiwalled carbon nanotubes (MWCNT) from three different sources and with different loadings has been compared, with a view to sensor applications. Additionally, poly(neutral red) (PNR) was electrosynthesised by potential cycling on bare glassy carbon and on MWCNT-modified glassy carbon electrodes, and characterised by cyclic voltammetry and scanning electron microscopy. Well-defined voltammetric responses were observed for hexacyanoferrate (II) oxidation with differences between the MWCNT types as well as from loading. The MWCNT and PNR/MWCNT-modified electrodes were applied to the oxidative determination of ascorbate, the electrocatalytic effects observed varying according to the type of nanotubes. Comparison was made with electrodes surface-modified by graphite powder. All modified electrode configurations with and without PNR were successfully employed for ascorbate oxidation at +0.05 V vs saturated calomel electrode with detection limits down to 4 μM; good operational stability and storage stability were also obtained.     

Effect of the nature of conductive supported nickel electrocatalyst for salicylic acid oxidation in alkaline medium

The electrocatalytic activity of Ni films electrodeposited on glassy carbon (Ni/GC), titanium (Ni/Ti), and gold (Ni/Au) electrodes toward salicylic acid (SA) oxidation are investigated. The cyclic voltammetry studies show that the nature of substrate strongly influences the apparent electrocatalytic activities of the nickel over layer in basic medium. It is observed that the Ni/GC electrode has higher activity for SA oxidation compared to other electrodes. Effects of various parameters such as concentration of Ni²⁺, deposition time for Ni film growth, and deposition potential on the electrooxidation of SA are investigated. It is demonstrated that the Ni(OH)₂/NiOOH plays the key role in the electrooxidation of SA. The response to SA on the Ni/GC electrode is examined using chronoamperometry.     

Functionalized mesoporous silica films as a matrix for anchoring electrochemically active guests

Mesoporous silica thin films were shown to be an appropriate matrix for immobilization of discrete electroactive moieties, yielding uniform transparent thin film electrodes with defined texture and enhanced electrochemical activity. The mesoporous silica films prepared on conducting FTO-coated glass substrate were postsynthetically functionalized. Alkoxysilanes were used as precursors for subsequent grafting via ionic or covalent bonds of representative electroactive species, such as polyoxometalate PMo12O(40)3-, hexacyanoferrate(III), and ferrocene. The electrochemically active concentration within the silica-based composite electrodes achieves 90, 260, and 60 micromol cm(-3) for polyoxometalate, hexacyanoferrate(III), and ferrocene, respectively. The amount of molecules involved in the charge-transfer sequence is proportional to the film thickness and comparable to the total amount of embedded guests. Thus, eventually the whole bulk volume of the modified silica films is electrochemically accessible. Immobilization in the chemically modified silica matrix alters the redox potential of the electroactive molecules. Electron exchange between the adjacent redox centers (electron hopping) is proposed as a possible charge propagation pathway through the insulating silica matrix, which is supported by the fact that the high charge uptake is observed also for the hybrid electrodes with the covalently anchored redox guests.