Determination of formal potential of NADH/NAD+ redox couple and catalytic oxidation of NADH using poly(phenosafranin)-modified carbon electrodes

The electrochemical regeneration of NADH/NAD⁺ redox couple has been studied using poly(phenosafranin) (PPS)-modified carbon electrodes to evaluate the formal potential and catalytic rate constant for the oxidation of NADH. The PPS-modified electrodes were prepared by electropolymerization of phenosafranin onto different carbon substrates (glassy carbon (GC) and basal-plane pyrolytic graphite (BPPG)) in different electrolytic solutions. The formal potential was estimated to be ? 0.365 ± 0.002 V vs. SHE at pH 7.0. As for the bare carbon electrodes, the oxidation of NADH at the BPPG electrode was found to be enhanced compared with the GC electrode. For the PPS-modified electrodes, it was found that the electrocatalysis of PPS-modified electrodes for the oxidation of NADH largely depends on the carbon substrate and electrolyte solution employed for their preparation, i.e., the PPS-modified BPPG electrode prepared in 0.2 M NaClO₄/acetonitrile solution exhibits an excellent and persistent electrocatalytic property toward NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 740 and 670 mV compared with those at the bare GC electrode and the PPS-modified GC electrode prepared in 0.2 M H₂SO₄ solution, respectively. A quantitative analysis of the electrocatalytic reaction based on rotating disk voltammetry gave the electrocatalytic reaction rate constants of the order of 10³–10⁴ M^?1 s^? 1 depending on the preparation conditions of the PPS-modified electrodes.     

Anodic electrochemiluminescent behavior of lucigenin on MWNT/GCE

The electrochemiluminescent (ECL) behavior of lucigenin on a multiwall carbon nanotubes modified glassy carbon electrode (MWNT/GCE) during anodic scanning was studied. A strong and stable anodic ECL signal was found on MWNT modified electrode, which results from the oxidation reaction between lucigenin and the oxidation production of OH^-. The effects of electrode materials, pH and scan rate on the ECL intensity were studied, and the possible ECL mechanism was also proposed. Under the optimized conditions, the ECL intensity was found to be linear with concentration of lucigenin in the range of 5.0 × 10^?7–5.0 × 10^?6 mol/L with a detection limit of 2.0 × 10^?7 mol/L. Superoxide dimutase (SOD) was found to be able to inhibit this ECL system, based on which a sensitive ECL methods for detection of SOD had been established.     

Electrochemical oxidation behavior of nitrite on a chitosan-carboxylated multiwall carbon nanotube modified electrode

A novel chitosan-carboxylated multiwall carbon nanotube modified glassy carbon electrode (MC/GCE) was developed to investigate the oxidation behavior of nitrite using cyclic voltammetry and differential pulse voltammetry modes. The electrochemical mechanism of the MC/GCE towards nitrite was discussed. The MC/GCE exhibited fast response towards nitrite with a detection limit of 1 × 10⁻⁷ mol l⁻¹ and a linear range of 5 × 10⁻⁷–1 × 10⁻⁴ mol l⁻¹. The possible interference from several common ions was tested. The proposed method was successfully applied in the detection of nitrite in real samples.     

NADH oxidation on screen-printed electrode modified with a new phenothiazine diazonium salt

NADH oxidation catalysts are extremely important in the field of electrochemical biosensors and enzymatic biofuel cells. Based on the growing diazonium chemistry, we synthesized the diazonium salt of the well-known NADH mediator toluidine blue O. The electrochemical reduction of the diazonium moiety by cyclic voltammetry onto a screen-printed electrode leads to an electrocatalyst suitable for the oxidation of NADH. The amperometric response for its oxidation shows a maximal current of 1.2 μA ([NADH] = 100 μM). Based on electrochemical measurements, the surface coverage is found to be 3.78 × 10^? 11 mol cm^? 2 and the heterogeneous standard rate constant k_h is 1.21 ± 0.16 s^? 1. The sensitive layer for the oxidation of NADH is improved by electrografting the diazonium salt with a potentiostatic method. Both the surface coverage and the heterogeneous standard rate constant k_h are improved and found to be 6.08 ± 0.63 × 10^? 11 mol cm^? 2 and ~ 5.02 s^? 1, respectively. The amperometric response is also improved by an 8 fold factor, reaching 9.87 μA ([NADH] = 120 μM). These remarkably high values for screen-printed electrodes are comparable to glassy carbon electrodes making this method suitable for low-cost bioelectronical devices.     

Electrochemiluminescent biosensor based on multi-wall carbon nanotube/nano-Au modified electrode

The electrochemiluminescent (ECL) behavior of lucigenin on a multi-wall carbon nanotube/nano-Au modified glassy carbon electrode (MWNT/nano-Au/GCE) was studied in this paper. Compared with the bare GCE, the ECL intensity of lucigenin can be greatly enhanced at MWNT/nano-Au/GCE. Based on the fact that superoxide dimutase (SOD) could obviously inhibit the ECL of lucigenin at MWNT/nano-Au/GCE, a sensitive ECL biosensor for determination of SOD was developed with a wide linear range of 5.0 × 10⁻⁸–5.0 × 10⁻⁶ mol/L with detection limit of 2.5 × 10⁻⁸ mol/L.     

Sodium dodecyl sulfate modified carbon nanotubes paste electrode as a novel sensor for the simultaneous determination of dopamine,ascorbic acid,and uric acid

A novel modified multiwall carbon nanotubes paste electrode with sodium dodecyl sulfate as a surfactant (SDS) has been fabricated through an electrochemical oxidation procedure and was used to electrochemically detect dopamine (DA), ascorbic acid (AA), uric acid (UA), and their mixture by cyclic voltammetry (CV) and differential voltammetry (DPV) methods. Several factors affecting the electrocatalytic activity of the hybrid material, such as the effect of pH, of the scan rate and of the concentration were studied. The bare carbon nanotubes paste electrode (BCNTPE) and SDS-modified carbon nanotubes paste electrode (SDSMCNTPE) were characterized using Field Emission Scanning Electron Microscopy (FESEM) and Energy-Dispersive X-ray spectroscopy (EDX). Using the CV procedure, a linear analytical curve was observed in the 1 × 10⁻⁶–2.8 × 10⁻⁵ M range with a detection limit at 3.3 × 10⁻⁷ M in pH 6.5, 0.2 M phosphate buffer solutions (PBS).     

A novel sensitive biosensor for Ca2+ based on electropolymerized melatonin modified electrode

An electropolymerized melatonin modified glassy carbon electrode (EPMT/GCE) was prepared by electrochemically polymerizing of melatonin in a 0.04 mol l^?1 perchloric acid solution. In KCl solution, Ca²⁺ was found to perform a reversible complex reaction on the EPMT/GCE to produce a sharp complex adsorption wave. The configuration of the electropolymerized membrane was characterized by scanning electron microscope and various electrochemical techniques. Based on which, a novel sensitive Ca²⁺ electrochemical sensor was established. The peak current showed a linear relationship with Ca²⁺ concentration in range of 6.2 × 10^?7–1.0 × 10^?4 mol l^?1 with detection limit of 4.5 × 10^?7 mol l^?1. The developed electrochemical sensor has been used for determination of Ca²⁺ in body fluid.     

Determination of nitrite with the electrocatalytic property to the oxidation of nitrite on thionine modified aligned carbon nanotubes

A thionine modified aligned carbon nanotubes (ACNTs) electrode was fabricated and was used to electrochemically determine nitrite. The thionine modified ACNTs electrode exhibited enhanced electrocatalytic behavior to the oxidation of nitrite. The electrochemical mechanism of the thionine/ACNTs electrode towards the oxidation of nitrite was discussed. The thionine modified ACNTs electrode exhibited fast response towards nitrite with a detection limit of 1.12 × 10⁻⁶ mol  L⁻¹ and a linear range of 3 × 10⁻⁶ – 5 × 10⁻⁴ mol  L⁻¹. The proposed method was successfully applied in the detection of nitrite in real samples.     

A voltammetric study of the binding of copper(II) to peptide fragments of prion

Binding of copper to three peptide fragments of prion (Cu²⁺ binding sites: 60–91, 92–96 and 180–193 amino acid residues) was investigated by anodic stripping voltammetry to determine the stoichiometries of Cu²⁺-prion peptide interactions. The method relies on the synthesis of N-terminally acetylated/C-terminally amidated peptide fragments of prion by solid-phase synthesis and direct monitoring of the oxidation current of copper in the absence and presence of each prion fragment. Titration curves of Cu²⁺ with Ac-PHGGGWGQ-NH₂, Ac-GGGTH-NH₂ and Ac-VNITKQHTVTTTT-NH₂ were obtained in concentrations ranging from 8.52 × 10^?7 to 5.08 × 10^?6, 3.95 × 10^?7 to 1.94 × 10^?6 and 7.82 × 10^?8 to 4.51 × 10^?7 M, respectively. The acquired data were used to calculate the stoichiometries (one peptide per Cu²⁺ ion for all the three studied systems) and apparent dissociation constants (K_d = 4.37 × 10^?8–3.50 × 10^?10 M) for the three complexes.     

Poly-xanthurenic acid as an efficient mediator for the electrocatalytic oxidation of NADH

This paper describes, for the first time, the development of a simple and highly sensitive chemical sensor based on a new electroactive material, electrogenerated in situ from xanthurenic acid on an electrode modified with MWCNT. The modified electrode shows efficient electrocatalytic oxidation activity towards NADH at an applied potential of 0.1 V vs. Ag/AgCl. The kinetic constant, k_kin, for the electrocatalytic oxidation of NADH, evaluated by chronoamperometry and voltammetry using RDE, provided values close to 10⁵ mol^?1 L s^?1.     

Porous cuprous oxide microcubes for non-enzymatic amperometric hydrogen peroxide and glucose sensing

Using porous cuprous oxide (Cu₂O) microcubes, a simple non-enzymatic amperometric sensor for the detection of H₂O₂ and glucose has been fabricated. Cyclic voltammetry (CV) revealed that porous Cu₂O microcubes exhibited a direct electrocatalytic activity for the reduction of H₂O₂ in phosphate buffer solution and the oxidation of glucose in an alkaline medium. The non-enzymatic amperometric sensor used in the detection of H₂O₂ with detection limit of 1.5 × 10^?6 M over wide linear detection ranges up to 1.5 mM and with a high sensitivity of 50.6 μA/mM. This non-enzymatic voltammetric sensor was further utilized in detection of glucose with a detection limit of 8.0 × 10^?7 M, a linear detection range up to 500 μM and with a sensitivity of ?70.8 μA/mM.     

The determination of cysteine at Bi-powder carbon paste electrodes by cathodic stripping voltammetry

The determination of cysteine by means of square wave cathodic stripping voltammetry (SWCSV) is reported here for the first time at Bi-modified carbon paste electrodes (CPEs). The modified electrodes are 17% w/w metallic Bi powder mixed with CP (Bi-CPEs) and the technique is based on the enhancement of Bi surface oxidation in the presence of cysteine at a carefully chosen accumulation potential and the subsequent reduction-stripping of the product (proposed to be bismuth(III) cysteinate) by potential scanning to more negative values. The wide concentration range of 1 × 10⁻⁶–5 × 10⁻⁵ M for cysteine can be assessed by SWCSV using Bi-CPEs and, by appropriate choice of accumulation times, two linear response concentration regimes could be identified: 1 × 10⁻⁶–1 × 10⁻⁵ M (accumulation for 600 s) and 1 × 10⁻⁵–5 × 10⁻⁵ M (accumulation for 100 s), with estimated detection limits of 3 × 10⁻⁷ and 2 × 10⁻⁶ M, respectively.     

Mineralization of aqueous phenolate solutions: A combination of irradiation treatment and wet oxidation

Wet oxidation (high-temperature, high-pressure oxidation of organic wastes in aqueous solution) and radiation technology were combined in γ-ray and electron beam induced oxidation of 4×10^?4–1×10^?2 mol dm^?3 Na-phenolate solutions in a wide O₂ concentration (1–20 bar pressure) and absorbed dose (0–50 kGy) range. Most experiments were made in stainless steel high pressure autoclave equipped with magnetic stirrer. The rate of oxidation was followed by chemical oxygen demand and total organic carbon content measurements. The rate was similar in γ-ray and pulsed electron beam irradiation and increased with O₂ concentration in the liquid.     

Direct electron transfer-type four-way bioelectrocatalysis of CO2/formate and NAD+/NADH redox couples by tungsten-containing formate dehydrogenase adsorbed on gold nanoparticle-embedded mesoporous carbon electrodes modified with 4-mercaptopyridine

Tungsten-containing formate dehydrogenase from Methylobacterium extorquens AM1 (FoDH1) catalyzes formate oxidation with NAD⁺. FoDH1 shows little direct communication with carbon electrodes, including mesoporous Ketjen Black-modified glassy carbon electrode (KB/GCE); however, it shows well-defined direct electron transfer (DET)-type bioelectrocatalysis of carbon dioxide reduction, formate oxidation, NAD⁺ reduction, and NADH oxidation on gold nanoparticle (AuNP)-embedded KB/GCE treated with 4-mercaptopyridine. Microscopic measurements reveal that the AuNPs (d = 5 nm) embedded on the KB surface are uniformly dispersed. Electrochemical data indicate that the pyridine moiety on the AuNPs plays important roles in facilitating the interfacial electron transfer kinetics and increasing the probability of productive orientation of FoDH1. The formal potential of the electrochemical communication site, which is most probably an ion‑sulfur cluster, is evaluated as − 0.591 ± 0.005 V vs. Ag | AgCl | sat. KCl from Nernst analysis of the steady-state catalytic waves.     

Selective determination of trichloroacetic acid using silver nanoparticle coated multi-walled carbon nanotubes

Silver nanoparticle coated multi-walled carbon nanotubes (Ag/MWCNT) were prepared and used to fabricate a modified electrode. The Ag/MWCNT composites were observed by a transmission electron microscope (TEM), and the electrochemical properties of the Ag/MWCNT composite modified glassy carbon electrode were characterized by electrochemical measurements. The results showed that these composites had a favorable catalytic ability for the reduction of trichloroacetic acid (TCAA). Square wave voltammetric (SWV) technique was applied to detect TCAA. Under optimum conditions, the voltammetric determination of TCAA was performed with a linear range of 5.0 × 10^? 6–1.2 × 10^? 4 mol L^? 1 and a detection limit of 1.9 × 10^? 6 mol L^? 1 (S/N = 3).     

Choline biosensors based on a bi-electrocatalytic property of MnO2 nanoparticles modified electrodes to H2O2

An interesting mode of reactivity of MnO₂ nanoparticles modified electrode in the presence of H₂O₂ is reported. The MnO₂ nanoparticles modified electrodes show a bi-direction electrocatalytic ability toward the reduction/oxidation of H₂O₂. Based on this property, a choline biosensor was fabricated via a direct and facile electrochemical deposition of a biocomposite that was made of chitosan hydrogel, choline oxidase (ChOx) and MnO₂ nanoparticles onto a glassy carbon (GC) electrode. The biocomposite is homogeneous and easily prepared and provides a shelter for the enzyme to retain its bioactivity. The results of square wave voltammetry showed that the electrocatalytic reduction currents increased linearly with the increase of choline chloride concentration in the range of 1.0 × 10⁻⁵ –2.1 × 10⁻³ M and no obvious interference from ascorbic acid and uric acid was observed. Good reproducibility and stability were obtained. A possible reaction mechanism was proposed.     

Flow injection potentiometric sensor for determination of phenylpropanolamine hydrochloride

A new polymeric membrane electrode has been constructed for the determination of phenylpropanolamine hydrochloride. The electrode was prepared by solubilizing the phenylpropanolamine-phosphomolybdate ion associate into a polyvinyl chloride matrix plasticized by dibutylphthalate as a solvent mediator. The electrode showed near-Nernstian response over the concentration range of 1 × 10^?5–1 × 10^?2 M with low detection limit of 6.3 × 10^?6 M. The electrode displays a good selectivity for phenylpropanolamine with respect to a number of common inorganic and organic species. The electrode was successfully applied to the potentiometric determination of phenylpropanolamine ion in its pure state and its pharmaceutical preparation in batch and flow injection conditions.     

Selective determination of uric acid with DNA doped polymers modified carbon fiber microelectrode

New biocomposite materials, based on the incorporation of DNA doped p-aminobenzensulfonic acid, was fabricated by electrochemical method. A carbon fiber microelectrode modified by this thin film was fabricated for selective determination of uric acid (UA) in the presence of a larger amount of ascorbic acid (AA). It was found that the voltammetric oxidation peak separation between UA and AA is about 260 mV at the modified electrode. A linear response of the peak current versus the concentration was found in the range of 8 × 10⁻⁷–6 × 10⁻⁴ M with correlation coefficient of 0.9991 and the detection limit was 5 × 10⁻⁷ M (s/n = 3) at the 5 × 10⁻⁴ M AA. The presence of high concentration AA did not interference the determination. The electropolymerized film was characterized by SEM techniques. The modified electrode shows good sensitivity, selectivity and stability.     

Amperometric sensing of dopamine using a single-walled carbon nanotube covalently attached to a conical glass micropore electrode

A single-walled carbon nanotube (SWNT) is covalently attached to the interior surface of a conical glass micropore electrode (GME) to create a novel amperometric dopamine sensor (SWNT/NH-GME). The SWNT/NH-GME combines the advantages of excellent transport properties of the cone-shaped micropore with the characteristics of a SWNT, exhibiting a dramatic electrocatalytic effect on the oxidation of dopamine (DA). Cyclic voltammetry and amperometric methods were employed to study the electrochemical behavior of the SWNT/NH-GME. The results showed that the SWNT/NH-GME sensor exhibited an excellent immunity from ascorbic acid interference and was able to measure DA concentrations with a detection limit of 4.2 × 10^?7 mol/L (S/N = 3).     

Direct electrochemistry of hemoglobin and its electrocatalytic effect based on its direct immobilization on carbon ionic liquid electrode

Direct electrochemistry of hemoglobin (Hb) has been achieved by its direct immobilization on carbon ionic liquid electrode (CILE). CILE was immersed in a solution containing Hb and ionic liquid, octylpyridinium chloride ([OcPy][Cl]), to directly immobilize Hb on CILE. Cyclic voltammetry of modified electrode exhibited quasi-reversible peaks corresponding to Hb. The oxidation and reduction peak potentials of immobilized Hb in acetate buffer solution, pH 5.0 and at a scan rate of 0.1 V s⁻¹ were obtained at about –150 mV and –290 mV, respectively. The average surface coverage of the electroactive Hb adsorbed on the electrode surface was calculated as 8.4 × 10⁻¹¹ mol cm⁻². Hb retained its bioactivity on modified electrode and showed excellent electrocatalytic activity towards oxygen, hydrogen peroxide and nitrite. Hydrogen peroxide can be determined in the range of 1.0 × 10⁻⁴–5.0 × 10⁻³ M.