Electrochemical redox pattern and allied interactive behaviour of FAD on a ruthenium-modified glassy carbon electrode surface

The redox and interactive behaviour of flavin adenine dinucleotide (FAD) with a ruthenium (Ru)-modified glassy carbon electrode (GCE) was investigated. The electron-transfer kinetics on the Ru-modified GCE gives an apparent electron-transfer coefficient, α _app of 0.56, and an apparent heterogeneous electron transfer rate constant, k _app of 2.32?s^?1, respectively. The cyclic voltammetry (CV) complemented by alternating cyclic voltammetry (ACV) shows reduction of FAD to be a quasi-reversible reaction involving FAD adsorption. The adsorption of FAD on the Ru-modified GCE fits a Langmuir adsorption isotherm. The large apparent negative Gibbs energy of adsorption ΔG _ads (?38.2?kJ?mol^?1) of FAD onto the Ru-modified GCE confirmed a strong chemical adsorption of FAD on the surface. The deposited Ru islands block surface sites for FAD adsorption and the electron-transfer communication between FAD and the electrode surface does not significantly improve with a deposited Ru monolayer.     

Preparation and Characterization of GCE Coatings Combining DDDMAB and PDADMAC with FAD and HCM for Electrocatalytic Detection of Oxygen and Sulfur Oxoanions

Electrochemically active hybrid coatings based on cationic films, didodecyldimethylammonium bromide (DDDMAB), and poly(diallyldimethylammonium chloride) (PDADMAC) are prepared on glassy carbon electrode surface by cycling the film‐covered electrode repetitively in a pH 7 solution containing flavin adenine dinucleotide (FAD), and anionic hexacyanometalate (HCM) complexes, Fe(CN)₆^3? and Ru(CN)₆^4?. Cyclic voltammetric features of hybrid coatings resemble that of electron transfer process of surface‐confined redox species. Electrochemical quartz crystal microbalance (EQCM) was used to monitor the deposition of FAD on DDDMAB film. Cyclic voltammetric peak potentials of modified electrode were found to be shifted to more negative region with increasing pH of contacting solution with a slope value of 63.3mV per pH unit. The electrocatalytic behavior of FAD‐modified DDDMAB‐coated GCE and hybrid film electrodes was tested towards reduction of oxygen, S₂O₈^2?, SO₅^2? and oxidation of SO₃^2?. The application of FAD‐modified DDDMAB‐coated GCE for S₂O₈^2? estimation was demonstrated in amperometric mode. The sensitivity and detection limit (S/N=3) were 267.6 μA mM^?1 and 2×10^?6 M, respectively.     

Modification of glassy carbon electrode with a polymer/mediator composite and its application for the electrochemical detection of iodate

A modified glassy carbon electrode was prepared by depositing a composite of polymer and mediator on a glassy carbon electrode (GCE). The mediator, flavin adenine dinucleotide (FAD) and the polymer, poly(3,4-ethylenedioxythiophene) (PEDOT) were electrochemically deposited as a composite on the GCE by applying cyclic voltammetry (CV). This modified electrode is hereafter designated as GCE/PEDOT/FAD. FAD was found to significantly enhance the growth of PEDOT. Electrochemical quartz crystal microbalance (EQCM) analysis was performed to study the mass changes in the electrode during the electrodeposition of PEDOT, with and without the addition of FAD. The optimal cycle number for preparing the modified electrode was determined to be 9, and the corresponding surface coverage of FAD (Γ_FAD) was ca. 5.11 × 10⁻¹⁰ mol cm⁻². The amperometric detection of iodate was performed in a 100 mM buffer solution (pH 1.5). The GCE/PEDOT/FAD showed a sensitivity of 0.78 μA μM⁻¹ cm⁻², a linear range of 4–140 μM, and a limit of detection of 0.16 μM for iodate. The interference effects of 250-fold Na⁺, Mg²⁺, Ca²⁺, Zn²⁺, Fe²⁺, Cl⁻, NO₃⁻, I⁻, SO₄²⁻ and SO₃²⁻, with reference to the concentration of iodate were negligible. The long-term stability of GCE/PEDOT/FAD was also investigated. The GCE/PEDOT/FAD electrode retained 82% of its initial amperometric response to iodate after 7 days. The GCE/PEDOT/FAD was also applied to determine iodate in a commercial salt.     

Adsorption phenomena in the NAD+/NADH system at glassy carbon electrodes

A variety of electrochemical approaches has been used to investigate the adsorption of NAD⁺, NADH and the NAD-NAD dimer from aqueous solution at glassy carbon electrodes (GCE) with supplementary studies of adsorption at pyrolytic graphite and platinum electrodes from aqueous media and at GCE from DMSO solution. The following hypotheses are advanced concerning the adsorption orientation: at carbon electrodes, on which NADH is not adsorbed, NAD⁺ produced by anodic oxidation of the NADH is first rapidly adsorbed in a planar configuration relative to the electrode surface, which is probably bound to the surface through the adenine moiety; there is then a relatively slow reorientation of the adsorbed NADH molecules to a perpendicular orientation relative to the electrode surface, which adsorbate is more tightly bound to the surface than the planar oriented adsorbate and which likely involves interaction between parallel adenine and pyridinium rings. Reduction (one-electron process) of NAD⁺ at the GCE produces the NAD-NAD dimer, which, at a clean electrode surface, involves a diffusion-controlled process and an adsorption-controlled process; the latter is due to formation of adsorbed dimer, which is more strongly adsorbed than NAD⁺. The dimer is oxidized at the GCE only if it is adsorbed. The factors controlling and involved in the adsorption processes have been examined with particular reference to the use of anodic voltammetry for the analytical determination of NADH.     

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.     

480—Redox and adsorption behaviour of some redox coenzymes involved in an electron-transport chain studied by the electro-optical technique

The redox and adsorption behaviour of some redox coenzymes involved in an electron-transport chain, i.e. ubiquinone-10 (CoQ), flavocoenzymes [flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD)] and nicotinamide adenine dinucleotide (NAD⁺), has been studied at a gold electrode by cyclic voltammetry and specular reflectivity measurement. All the coenzymes investigated were found to participate in electron transport in adsorbed states on the electrode surface. Adsorbed CoQ and flavocoenzymes are reduced and the resulting products remain adsorbed at the surface. Contrary to them, adsorbed NAD⁺ is reduced and then desorbed immediately. Possible models for the surface orientation of adsorbed molecules were proposed based on the experimental data.Some analogies can be noted between the interfacial behaviour of these coenzymes at the electrode and that in mitochondria.     

Modeling the surface phenomena in carbon paste electrodes by low frequency impedance and double-layer capacitance measurements

Impedance and capacitance studies have been performed with covalently coupled Glucose oxidase (GOD) enzyme, covalently coupled flavin adenine dinucleotide (FAD), reconstituted GOD enzyme and blank carbon paste electrodes to study the changes in the electrochemical interfacial properties. Impedance studies were performed using a low frequency impedance technique and the electrochemical surface capacitance was measured by a pulse technique. We have attempted to fit the experimental values to an equivalent circuit model. The Randles' cell circuit with Warburg impedance modeled well the experimental values and the behavior of the enzyme electrodes. The individual components of the model were calculated and the parameters were explained. The blank paste electrode showed a constant phase element behavior.     

腺嘌呤电化学氧化及反应速度常数测定

腺嘌吟(6-氨基嘌呤)的初级电解氧化过程即由化合物Ⅰ氧化至化合物Ⅱ是一个多步的、高度不可逆的六电子转移过程已为周期伏安法(简称CV实验所证实,其限速步骤电子转移数n_b=2^[1],在线性扩散条件下,完全不可逆电极过程CV氧化峰电位时的异相反应速度常数为^[2]。     

SPONTANEOUS LOW-LEVEL CHEMILUMINESCENCE FROM CHEMICAL AND ELECTROCHEMICAL OXIDATION OF β-DEHYDRO NICOTINEAMIDE ADENINE DINUCLEOTIDE

Abstract— Oxidation of β-dehydro nicotineamide adenine dinucleotide (NADH) by oxygen catalyzed by horseradish peroxidase was accompanied by spontaneous, low-level chemiluminescence in the existence of co-oxidant 2,4-dichlorophenol. Electrochemical oxidation of NADH by the glassy carbon electrode also emitted a spontaneous low-level chemiluminescence above the potental +0.7 V vs reference electrode (Ag/AgCl) in aqueous medium in pH 7.0˜10.0. Low-level electrochemilumin-escence was observed when the NAD and oxygen species coexisted in the neighborhood of the electrode surface. The main photon emission process was supposed to be generated by the interaction of pyridine moiety of NAD and oxygen.     

Manganese Oxide Nanoparticles/Reduced Graphene Oxide as Novel Electrochemical Platform for Immobilization of FAD and its Application as Highly Sensitive Persulfate Sensor

In this study, manganese oxide nanoparticles/reduced graphene oxide(MnOxNPs/rGO) was used as support for strong immobilization of flavin adenine dinucleotide(FAD). A thin film of rGO cast on the electrode surface, followed by performing electrodeposition of MnOxNPs at applied constant potential of +1.4 V vs. Ag/AgCl for 200 s. Finally, FAD was electrodeposited onto the rGO/MnOxNPs film by potential cycling between 1.0 to ?1.0 V in solution containing 1 mg ml^?1 FAD. Electrochemical properties and catalytic activity of GCE/rGO‐MnOxNPs/FAD toward persulfate (S₂O₈^2?) reduction was investigated. Under optimized condition, the concentration calibration range, detection limit, and sensitivity were 0.1 μM–2 mM, 90 nM and 125.8 nA/μM, respectively, using hydrodynamic amperometry technique.     

Amperometric Ethanol Biosensor Based on Integration of Alcohol Dehydrogenase with Meldola's Blue/Ordered Mesoporous Carbon Electrode

An amperometric ethanol biosensor was fabricated by integration of alcohol dehydrogenase (ADH) with meldola's blue (MB)/ordered mesoporous carbon (OMC) composite modified glassy carbon electrode (MB/OMC/GCE). The MB/OMC/GCE was highly sensitive for nicotinamide adenine dinucleotide (NADH) measurement (9.1±0.25 μA/mM) and gave a low detection limit of 0.21±0.02 μM. The ethanol biosensor exhibited a wide linear range up to 6 mM with a lower detection limit of 19.1±0.58 μM as well as a high sensitivity of 34.58±2.43 nA/mM without suffering any interference from some common electroactive compounds.     

硫堇-甲苯二异氰酸酯衍生物修饰的玻碳基乙醇传感器的研究

硫堇－甲苯二异氰酸酯衍生物修饰的玻碳基乙醇传感器的研究俞爱民,韩吉林,杨可盛,陈洪渊（南京大学化学系,南京,２１００９３）关键词硫堇－ＴＤＩ衍生物,ＮＡＤ~＋,ＡＤＨ,乙醇传感器经典的测定乙醇含量的方法有气相色谱法［１］、分光光度法［２］．近年来,随?..     

Direct electrochemistry of DNA,guanine and adenine at a nanostructured film-modified electrode

A nanostructured film electrode, a multi-wall carbon nanotubes (MWNT)-modified glassy carbon electrode (GCE), is described for the simultaneous determination of guanine and adenine. The properties of the MWNT-modified GCE were investigated by scanning electron microscopy (SEM) and cyclic voltammetry. The oxidation peak currents of guanine and adenine increased significantly at the MWNT-modified GCE in contrast to those at the bare GCE. The experimental parameters were optimized and a direct electrochemical method for the simultaneous determination of guanine and adenine was proposed. Using the MWNT-modified GCE, a sensitive and direct electrochemical technique for the measurement of native DNA was also developed, and the value of (G+C)/(A+T) of HCl-digested DNA was detected.     

多璧碳管/聚中性红修饰电极的制备及对鸟嘌呤和腺嘌呤的应用研究

结合纳米材料的电催化特性和中性红聚合物薄膜的分子识别能力, 以玻碳电极为基体制备了多壁碳管/聚中性红(MWNT/PNR)修饰电极, 并用表面扫描电镜和循环伏安法进行了表征. 实验表明, 该修饰电极对腺嘌呤(A)和鸟嘌呤(G)都表现出了良好的电催化性能. 在最佳条件下, 用示差脉冲伏安法对A和G进行了测定, 其氧化峰电流于A和G的浓度分别在0.01～4 μmol/L和0.01～8 μmol/L范围内呈良好的线性关系, 检测限均为5×10-9 mol/L (S/N=3). 该修饰电极可以用来同时测定DNA中的A和G.     

Indirect electrochemical reduction of nicotinamide coenzymes

Nicotinamide coenzymes nicotinamide adenine dinucleotide (NAD(+)) and nicotinamide adenine dinucleotide phosphate (NADP(+)) were electrochemically reduced to NADH and NADPH, respectively. As direct reduction of nicotinamide coenzymes leads to inactive by-products, an indirect method using (pentamethylcyclopentadienyl-2,2'-bipyridine aqua) rhodium (III) as the mediator, was applied. A phosphate buffer solution, pH 8, with 1-10 mM NAD(P)(+) and 2.5-200 microM mediator, was pumped through a glassy carbon packed bed cathode. Virtually all the NAD(P)(+) was reduced to NAD(P)H in the cell. No sign of mediator loss due to side-reactions was detected though the mediator molecules shuttled hundreds of times between the oxidised and the reduced form. Adsorption of mediator molecules on the surface of the carbon cathode was found to be important for the reduction process. Due to strong adsorption, only minute amounts of mediator were consumed.     

One-step construction of an electrode modified with electrodeposited Au/SiO2 nanoparticles, and its application to the determination of NADH and ethanol

A new electrode was developed by one-step potentiostatic electrodeposition (at ?2.0 V for 20 s) of Au/SiO₂ nanoparticles on a glassy carbon electrode. The resulting electrode (nano-Au/SiO₂/GCE) was characterized by scanning electronic microscopy, X-ray photoelectron spectroscopy and electrochemical techniques. The electrochemical behavior of dihydronicotinamide adenine dinucleotide (NADH) at the nano-Au/SiO₂/GCE were thoroughly investigated. Compared to the unmodified electrode, the overpotential decreased by about 300 mV, and the current response significantly increased. These changes indicated that the modified electrode showed excellent catalytic activity in the oxidation of NADH. A linear relationship was obtained in the NADH concentration range from 1.0?×?10^?6 to 1.0?×?10^?4 mol?L^?1. In addition, amperometric sensing of ethanol at the nano-Au/SiO₂/GCE in combination with alcohol dehydrogenase and nicotinamide adenine dinucleotide was successfully demonstrated. A wide linear response was also found for ethanol in the range from 5.0?×?10^?5 to 1.0?×?10^?3 mol?L^?1 and 1.0?×?10^?3 to 1.0?×?10^?2 mol?L^?1, respectively. The method was successfully applied to determine ethanol in beer and biological samples.     

In situ Electroreduction of Graphene Oxide: Increased Sensitivity for the Determination of NADH

A novel and useful method to catalyze the electro‐oxidation of nicotinamide adenine dinucleotide (NADH) over a glassy carbon electrode (GCE) modified with graphene oxide (GO) is presented. Based on the presence of oxygen moieties in GO, which can be easily reduced, an in situ electrochemical generation of reduced graphene oxide (denoted as erGO) applying a sufficient negative potential. A potential of ?1.000 V was selected to generate the erGO/GCE as a pretreatment potential before the detection of NADH. The in situ generated erGO/GCE system produces a decrease in the overpotential of NADH oxidation from +0.720 V to +0.230 V compared with GCE. The process also produced an important increase in current signals. The modified electrode was characterized by scanning electron (SEM) and electrochemical microscopies (SECM), cyclic voltammetry and by Raman spectroscopy. Amperometric detection of NADH via this straightforward electrocatalytic method provides a wide linear range between 10 and 100 μM, a lower detection limit of 0.36 μM and an excellent sensitivity of (1.47±0.09) μA mM^?1.     

Autofluorescence spectroscopy of normal and malignant human breast cell lines

The fluorescence of tryptophan, reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) and flavin adenine dinucleotide (FAD) were characterized in normal human breast cells as well as in malignant human breast cells of similar and dissimilar genetic origins. Fluorescence measurements of each cell line were made over a wide range of cell concentrations, and the fluorescence per cell was determined from the slope in the linear range of the fluorescence intensity vs cell concentration plot. All of the malignant cells showed a statistically significant decrease in the tryptophan fluorescence per cell relative to that of the normal cells. No statistically significant differences were observed in the NAD(P)H or FAD fluorescence per cell between the normal and any of the malignant cell types. NAD(P)H fluorescence was also imaged from monolayers of the normal and malignant cells (of similar genetic origin) using two-photon fluorescence microscopy. A statistically significant decrease in the NAD(P)H fluorescence with malignancy was observed, suggesting that fluorescence imaging of single cells or the cell monolayer preparation may provide more contrast than volume-averaged fluorescence measurements of cells in suspension. In conclusion, the differences in normal and malignant human breast tissue fluorescence spectra may be attributed in part to differences in the intrinsic cellular fluorescence of normal and malignant breast epithelial cells.     

Electrochemical property and electroanalytical application of large mesoporous carbons

This article reports on the electrochemical property of large mesoporous carbons (LMC) synthesized using nano-CaCO₃ as a template and sucrose as a carbon precursor. Though a simple preparation method for LMC, the LMC modified glassy carbon electrode (LMC/GCE) exhibited good electrochemical activity for some common electroactive compounds, such as ascorbic acid (AA), uric acid (UA), epinephrine (EP) and tryptophan (Trp). The unique properties of LMC were also compared with those of carbon nanotubes (CNT), and the results showed that the LMC possessed a much better property than CNT. In addition, the LMC/GCE was also used to analyze the β-nicotinamide adenine dinucleotide (NADH) and hydrogen peroxide (H₂O₂) and showed a high sensitivity and low detection limit. Based on a simple preparation method and good electrochemical property, the LMC represents a new class of carbon electrode for electrochemical sensor applications.     

Electrocatalytic reaction of hydrogen peroxide and NADH based on poly(neutral red) and FAD hybrid film

A simple method to immobilize poly(neutral red) (PNR) and flavin adenine dinucleotide (FAD) hybrid film (PNR/FAD) by cyclic voltammetry is proposed. The PNR/FAD hybrid film can be easily prepared on an electrode surface involving electropolymerization of neutral red (NR) monomers and the electrostatic interaction between the positively charged PNR and the negatively charged FAD. It exhibits electroactive, stable, surface-confined, pH-dependent, nano-sized, and compatible properties. It provides good electrocatalytic properties to various species. It shows a sensitivity of 5.4 μA mM(-1) cm(-2) and 21.5 μA mM(-1) cm(-2) for hydrogen peroxide (H(2)O(2)) and nicotinamide adenine dinucleotide (NADH) with the linear range of 0.1 μM-39 mM and 5 × 10(-5) to 2.5 × 10(-4) M, respectively. It shows another linear range of 48.8-355.5 mM with the sensitivity of 12.3 μA mM(-1) cm(-2) for H(2)O(2). In particular, the PNR/FAD hybrid film has potential to replace some hemoproteins to be a cathode of biofuel cells and provide the biosensing system for glucose and ethanol.