Nile blue adsorbed onto silica gel modified with niobium oxide for electrocatalytic oxidation of NADH

A study of modified carbon paste electrode employing Nile blue (NB) adsorbed on silica gel modified with niobium oxide (SN) for electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH) is described. The adsorbed organic dye on SN was used to prepare a modified carbon paste electrode to investigate its electrochemical properties. The formal potential (E°′) of the adsorbed NB (−230 mV vs. saturated calomel electrode, SCE) showed a shift of 70 mV towards a more positive potential value, compared to NB dissolved in aqueous solution. In solutions with pH between 6.0 and 8.0 did stability and E°′ remained almost constant. However, for a solution pH lower than 6.0 the E°′ was affected by the acidity of the contacting solution, shifting the E°′ towards more positive values. For the solution pHs between 6.0 and 8.0 the electrocatalytic activity remained almost constant. A linear response range for NADH between 1.0×10⁻⁵ and 5.2×10⁻⁴ mol l⁻¹, at pH 7.0, was observed for the electrode, with an applied potential of −200 mV versus SCE. The formation of an intermediate charge transfer (CT) complex was proposed to the CT reaction between NADH and adsorbed NB. The heterogeneous electron transfer rate, k_obs, was 1400 M⁻¹ s⁻¹ and the apparent Michaelis-Menten constant, was 0.21 mM at pH 7.0 evaluated from rotating disk electrode (RDE) experiments with an electrode coverage of about 5.2×10⁻⁹ mol cm⁻². The increase in the reaction rate between NADH and the immobilized NB compared to those obtained with dissolved NB was assigned to the shift of the E°′ towards more positive values.     

Electrocatalytic activity of 2,3,5,6-tetrachloro-1,4-benzoquinone/multi-walled carbon nanotubes immobilized on edge plane pyrolytic graphite electrode for NADH oxidation

This work reports the electrocatalytic activity of 2,3,5,6-tetrachloro-1,4-benzoquinone (TCBQ)/multi-walled carbon nanotubes (MWCNT) immobilized on an edge plane pyrolytic graphite electrode for nicotinamide adenine dinucleotide (NADH) oxidation. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) were used to confirms the presence of chloro after the nanotube modification with 2,3,5,6-tetrachloro-1,4-benzoquinone. The surface charge transfer constant, k_s, and the charge transfer coefficient for the modified electrode, α, were estimated as 98.5 (±0.6) s⁻¹ and 0.5, respectively. With this modified electrode the oxidation potential of the NADH was shifted about 300 mV toward a less positive value, presenting a peak current much higher than those measured on an unmodified edge plane pyrolytic graphite electrode (EPPG). Cyclic voltammetry and rotating disk electrode (RDE) experiments indicated that the NADH oxidation reaction involves 2 electrons and a heterogenous rate constant (k_obs) of 3.1 × 10⁵ mol⁻¹ l s⁻¹. The detection limit, repeatability, long-term stability, time of response and linear response range were also investigated.     

Electrochemical oxidation of 2′,3′-dideoxyadenosine at pyrolytic graphite electrode

The oxidation chemistry of 2′,3′-dideoxyadenosine (I) has been studied at a pyrolytic graphite electrode (PGE) in the pH range 2.4-10.9 and a well defined oxidation peak was noticed. The peak potential of the peak was linearly dependent on pH with dE_p/dpH as 42 mV/pH. Voltammetric, coulometric, spectral studies and product analysis indicate that the oxidation of (I) occurs in an EC reaction involving 6e⁻, 6H⁺ process at pH 7.2 to give allantoin, C-C dimer and dideoxyribose as the major products and a C-O-O-C linked dimer as a minor product. Tentative mechanisms for the formation of the products have also been suggested. A comparison of peak potential value of 2′,3′-dideoxyadenosine with adenosine and 2′-deoxyadensoine indicated that the difference is insignificant which has further been supported by the calculations of difference of energies of lowest unoccupied and highest occupied molecular orbitals.     

o-Phenylenediamine adsorbed onto silica gel modified with niobium oxide for electrocatalytic NADH oxidation

A study of a modified carbon paste electrode employing o-phenylenediamine (PDA) adsorbed onto silica gel modified with niobium oxide (SN) for electrocatalytic oxidation of nicotinamide adenine dinucleotide (NADH) is described. The species adsorbed on SN was used to prepare a modified carbon paste electrode to investigate its electrochemical properties. The formal potential (E^0′) of the adsorbed PDA was −140 mV vs. SCE (saturated calomel electrode). The electrochemical behavior of the adsorbed PDA, compared to that of PDA dissolved in aqueous solution, was completely different. In solution, pH between 3.0 and 8.0, E^0′ remained almost constant and the response was very stable. A linear response range for NADH between 4.0×10⁻⁵ and 8.0×10⁻⁴ mol l⁻¹, at pH 7.0, was observed for the electrode, with an applied potential of −50 mV vs. SCE. The formation of an intermediate charge transfer complex is proposed for the charge transfer reaction between NADH and adsorbed PDA. The heterogeneous electron transfer rate, k_obs, was 5480 mol⁻¹ l s⁻¹ and the apparent Michaelis-Menten constant, 1.04×10⁻⁴ mol l⁻¹ at pH 7.0, evaluated with rotating disk electrode (RDE) experiments with an electrode with a coverage of PDA of 5.7×10⁻⁹ mol cm⁻². The slight increase in the reaction rate with the solution pH was assigned to the thermodynamic driving force.     

A comparison of the electrocatalytic activities of ordered mesoporous carbons treated with either HNO3 or NaOH

Ordered mesoporous carbon (OMC) was treated with HNO₃ or NaOH. The two treated OMCs have many oxygen-containing functional groups. Those treated with HNO₃ have more acidic surface groups than those treated with NaOH. Nicotinamide adenine dinucleotide (NADH) and H₂O₂ were selected as marker molecules for the comparison of the electrocatalytic property of the OMCs. A comparison between the cyclic voltammograms shows that the oxidation peak potential of NADH is 0.614 V at a bare glassy carbon electrode (GCE), 0.205 V at OMC/GCE, 0.223 V at NaOH-treated OMC/GCE, and 0.0 V at HNO₃-treated OMC/GCE (vs. Ag/AgCl). The results indicate that the HNO₃-treated OMC/GCE exhibits the highest electrocatalytic activity for NADH oxidation. Thus, acidic groups rather than other oxygen-containing functional groups, play a very important role in the catalytic activity of OMC.     

Mediated oxidation of guanine by [Ru(bpy)2dpp] and their electrochemical assembly on the ITO electrode

Electrochemical oxidation of guanine mediated by [Ru(bpy)₂dpp]²⁺ (where bpy = 2,2′-bipyridine, dpp = 2,3-bis (2-pyridyl) pyrazine) and their electrochemical assembly at an ITO electrode prompted by guanine have been investigated with cyclic voltammetry and differential pulse voltammetry. It is found that [Ru(bpy)₂dpp]²⁺ can serve as an excellent mediator to induce the oxidation of guanine, and the mediated peak currents increase linearly with the rise of guanine concentration in the range from 0.01 to 0.20 mmol L⁻¹. Interestingly, with the increase of repetitive voltammetric sweeping numbers, [Ru(bpy)₂dpp]^3+/2+ can be assembled onto the ITO electrode and guanine has the ability to enhance the peak currents of prewaves. Also, with the rise of guanine concentration from 0.01 to 0.15 mmol L⁻¹, the peak currents of prewaves increase gradually. Meanwhile, the mediated mechanism of guanine oxidation by [Ru(bpy)₂dpp]²⁺ and the assembled process of [Ru(bpy)₂dpp]^3+/2+ on the ITO surface in the presence of guanine are discussed in detail.     

Modelling electrocatalysis of hydroquinone oxidation by nicotinamide adenine dinucleaotide coenzyme encapsulated within SBA-15 and MCM-41 mesoporous aluminosilicates

The electrochemical response of NADH associated to two mesoporous aluminosilicates, MCM-41 and SBA-15, is described upon attachment of such materials into polymer-film electrodes. The studied materials display a significant electrocatalytic activity towards the oxidation of 1,4-dihydrobenzoquinone, H₂Q. Two models for describing the electrocatalytic process, based on the general theory of mediated electrocatalysis and the Lovric and Scholz formulation of the voltammetry of microparticles are discussed. Voltammetric and chronoamperometric data indicate that the electrocatalytic process involves the formation of a surface-confined NADH-H₂Q adduct in the case of SBA-15, while a surface reaction/regeneration scheme prevails for MCM-41.     

Poly-phenothiazine derivative-modified glassy carbon electrode for NADH electrocatalytic oxidation

Electropolymerization of a new phenothiazine derivative (bis-phenothiazin-3-yl methane; BPhM) on glassy carbon (GC) electrode generates a conducting film of poly-BPhM, in stable contact with the electrode surface. The heterogeneous electron-transfer process corresponding to the modified electrode is characterized by a high rate constant (50.4 s⁻¹, pH 7). The GC/poly-BPhM electrode shows excellent electrocatalytic activity toward NADH oxidation. The rate constant for catalytic NADH oxidation, estimated from rotating disk electrode (RDE) measurements and extrapolated to zero concentration of NADH, was found to be 9.4 × 10⁴ M⁻¹ s⁻¹ (pH 7). The amperometric detection of NADH, at +200 mV vs. SCE, is described by the following electroanalytical parameters: a sensitivity of 1.82 mA M⁻¹, a detection limit of 2 μM and a linear domain up to 0.1 mM NADH.     

New Cu(I) complexes with 2,2′-biquinolyl and 2,2′-quinolyl-pyridine containing polymer ligands as electrocatalysts for O2 activation in the oxidation of aliphatic amines

A series of polynuclear Cu(I) complexes with various types of polymer ligands containing 2,2′-biquinolyl (biQ) or 2,2′-quinolyl-pyridine (QPy) fragments in the polymer backbone was synthesized using sacrificial Cu anode. Cyclic voltammetry investigation of the obtained complexes revealed the formation of two types of Cu(I) coordination units, depending on the polymer's structure and electrolysis conditions. The first type of complexes, with only one biQ ligand per Cu(I) center, showed high oxidase activity in the air oxidation of primary and secondary amines to corresponding aldehydes with concomitant reduction of molecular oxygen to water. The reactions proceed in acetonitrile or N-methylpyrrolidone in the presence of O₂ at a potential of Cu^II/Cu^I electroreduction (−0.55 V vs. Ag/AgCl/KCl) with the high preparative yield and current efficiency. The possible scheme of the electrocatalytic process is discussed.     

Meldola blue immobilized on a new SiO2/TiO2/graphite composite for electrocatalytic oxidation of NADH

Meldola blue immobilized on a new SiO₂/TiO₂/graphite composite was applied in the electrocatalytic oxidation of NADH. The materials were prepared by the sol-gel processing method and characterized by several techniques including scanning electronic microscopy coupled to energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electronic microscopy (HRTEM). Si and Ti mapping profiles on the surface showed a homogeneous distribution of the components. Ti2p binding energy peaks indicate that the formation of Si-O-Ti linkage is presumably the responsible for the high rigidity of the matrices. The good electrical conductivity presented by the composites (5 and 11 S cm⁻¹) can be related to a homogeneous distribution of graphite particles observed by TEM. After the materials characterization, a SiO₂/TiO₂/graphite electrode was prepared and some chemical modifications were performed on its surface to promote the adsorption of meldola blue. The resulting system presented electrocatalytic properties toward the oxidation of NADH, decreasing the oxidation potential to −120 mV. The proposed sensor showed a wide linear response range from 0.018 to 7.29 mmol l⁻¹ and limit of detection of 0.008 mmol l⁻¹. SiO₂/TiO₂/graphite has shown to be a promising material to be used as a suitable support in the construction of new electrochemical sensors.     

Electrocatalytic activity of Pt nanoparticles deposited on porous TiO2 supports toward methanol oxidation

Porous TiO₂ thin films were prepared on the Si substrate by hydrothermal method, and used as the Pt electrocatalyst support for methanol oxidation study. Well-dispersed Pt nanoparticles with a particle size of 5–7 nm were pulse-electrodeposited on the porous TiO₂ support, which was mainly composed of the anatase phase after an annealing at 600 °C in vacuum. Cyclic voltammetry (CV) and CO stripping measurements showed that the Pt/TiO₂ electrode had a high electrocatalytic activity toward methanol oxidation and an excellent CO tolerance. The excellent electrocatalytic performance of the electrode is ascribed to the synergistic effect of Pt nanoparticles and the porous TiO₂ support on CO oxidation. The strong electronic interaction between Pt and the TiO₂ support may modify CO chemisorption properties on Pt nanoparticles, thereby facilitating CO oxidation on Pt nanoparticles via the bifunctional mechanism and thus improving the electrocatalytic activity of the Pt catalyst toward methanol oxidation.     

Electrosynthesis and efficient electrocatalytic performance of poly(neutral red)/ordered mesoporous carbon composite

A novel composite film which contains ordered mesoporous carbon (OMC) along with the incorporation of poly(neutral red) (PNR) has been synthesized on glassy carbon electrode by potentiostatic method. This composite film was characterized by scanning electron microscope (SEM) and cyclic voltammetry (CV). Two pairs of the redox peaks appear at formal potential E^0′ = +0.045 V (peak I) and E^0′ = −0.49 V (peak II) at the PNR/OMC/GC electrode. And it is found that only the redox waves (peak I) exhibits good electrocatalytic activity towards nicotinamide adenine dinucleotide (NADH) and 2-mercaptoethanol (2-ME). Under a lower operation potential of +0.07 V, amperometry method was used to determine the concentration of NADH and 2-ME, respectively. In pH 7.0, sensors for two molecules under their corresponding optimized conditions were developed with acceptable sensitivity and low detection limits in large determination ranges. In addition, these sensors have good reproducibility and stability.     

Electrocatalytic oxidation of methanol at Ni–Al layered double hydroxide film modified electrode in alkaline medium

Ni–Al–NO₃ layered double hydroxides (LDH) are electrodeposited on the glassy carbon (GC) electrodes and the electrocatalytic activities of the modified electrodes toward methanol oxidation are studied in detail by cyclic voltammetry and chronoamperometry. Various factors affecting the electro-oxidation of methanol are investigated for optimizing the electrocatalytic properties and making the mechanism clearly such as methanol concentration, scan rate, KOH concentration, Ni:Al ratio of Ni–Al LDH used. The results show that Ni–Al–NO₃ LDH exhibit higher electrocatalytic activity for methanol oxidation and better stability than that of Ni(OH)₂ prepared under the same condition. The LDH with Ni:Al ratio of 3:1 display a good electrocatalytic activity for methanol oxidation in 0.5 M KOH. The mechanism of methanol oxidation on Ni–Al LDHf/GC electrode is also proposed according to the experimental results, involving in both a chemical oxidation via Fleischmann's mechanism and a direct electro-oxidation on the Ni³⁺ oxide surface.     

Amplification of electrocatalytic oxidation of NADH based on cysteine nanolayers

A biosensor for the determination of alcohol based on cysteine (Cys) nanolayers has been constructed. The utilization of the redox-inactive, self-assembled monolayer (SAM) modified gold ultra micro-electrode (UME) for NADH electroanalysis is introduced as a new modified gold Cys-UME. This Cys-UME prevents fouling and was found to be stable and ill-defined under subsequent potential sweeps, and different NADH concentrations. NADH oxidation gave a linear correlation between the peak current and NADH concentration (1–5 mM). Also, excellent electro-catalytic activity (enhancing the oxidation peak current) was observed towards NADH oxidation, with activation overpotential of about 250 mV lower than that of the bare electrode. The use of different alcohol concentrations in the presence of alcohol dehydrogenase (ADH) gave an increase in the NADH oxidation peak current, indicating that the electrochemical NADH oxidation at mild potentials could lead to an enzymatically active NAD⁺.     

Coulometric bioelectrocatalytic reactions based on NAD-dependent dehydrogenases in tricarboxylic acid cycle

This paper describes the characterization of mediated electro-enzymatic electrolysis systems based on NAD-dependent dehydrogenase reactions in the tricarboxylic acid (TCA) cycle. A micro-bulk electrolysis system with a carbon felt anode immersed in an electrolysis solution with a value of about 10 μL was constructed for coulometric analysis of the substrate oxidation. Diaphorase (DI) was used to couple the NAD-dependent dehydrogenase reaction with the anode reaction of a suitable redox mediator. We focused on three types of NAD-dependant dehydrogenases reactions in this research: (1) isocitrate oxidation, in which the standard Gibbs energy change (ΔG°′) is negative; (2) α-ketoglutarate oxidation, which involves an electrochemically active coenzyme A (CoA); and (3) malate oxidation, which is thermodynamically unfavorable because of a large positive ΔG°′ value. The complete electrolysis of isocitrate was easily achieved, supporting the effective re-oxidation of NADH in the diaphorase-catalyzed electrochemical reaction. CoA was unfavorably oxidized at the electrodes in the presence of some mediators. The electrocatalytic oxidation of CoA was suppressed and the quantitative electrochemical oxidation of α-ketoglutarate was achieved by selecting a suitable mediator with negligibly slow electron transfer kinetics with CoA. The uphill malate oxidation was susceptible to product inhibition in the bioelectrochemical system, although NADH generated in the malate dehydrogenase reaction was immediately oxidized in the electrochemical system. The inhibition was successfully suppressed by linking citrate synthase to quench oxaloacetate and to make the total ΔG°′ value negative.     

CeO2 nanoparticles decorated multi-walled carbon nanotubes for electrochemical determination of guanine and adenine

Sub-10 nm CeO₂ nanoparticles decorated multi-walled carbon nanotubes has been constructed for electrochemial determination of guanine and adenine. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize the nanoparticles CeO₂/MWCNTs. Electrochemical impedance spectroscopy (EIS) was used to characterize the electrode modifying process. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to study the electrocatalytic activity toward the electrochemical oxidation of guanine and adenine. The detection limit (S/N = 3) for adenine and guanine was found to be 20 and 10 nM, respectively. The obtained sensitivity toward guanine and adenine was 1.26 and 1.13 μA/μM in the linear concentration range 5–50 μM and 5–35 μM, respectively. These results demonstrate that the carbon nanotubes could provide huge locations and facilitate the adsorptive accumulation of the guanine and adenine, and the CeO₂ nanoparticles are promising substrates for the development of high-performance electrocatalysts for biosensing.     

The interaction of water-soluble iron porphyrins with DNA films and the electrocatalytic properties for inorganic and organic nitro compounds

The electrochemistry of water-soluble iron porphyrins (Fe(n-TMPyP)) (where n=2 and 4) was studied as an electrochemically active film on DNA modified glassy carbon, gold, platinum, and transparent semiconductor tin oxide electrodes in solutions of various pH values. The two layers of the modified electrode containing the iron porphyrin and the DNA film were prepared by depositing the iron porphyrin on a DNA film modified electrode. The Fe(4-TMPyP)/DNA film was electrocatalytic reductive for p-nitrobenzoic acid in a weak acidic, or neutral aqueous solution through an Fe^II species, and the electrocatalytic reduction peak potential became more negative than the cathodic peak of the Fe^III/II redox couple. The electrocatalytic reduction properties by the Fe(2-TMPyP)/DNA film as catalysts for nitrite reduction have also been determined, and shown to be active through an Fe^I species and to be pH-dependent. The electrocatalytic oxidation properties of nitrite by Fe(n-TMPyP)/DNA (for n=2 and 4) film have also been determined and shown to be active through an Fe^IV species with the electrocatalytic oxidation efficiency of NO₂⁻ with Fe^IV(O)(n-TMPyP) being higher than with (HO)Fe^IV(O)(n-TMPyP). The electrocatalytic oxidation efficiency of NO₂⁻ by iron porphyrin is pH-dependent. The electrocatalytic reduction of p-nitrophenol by Fe(2-TMPyP)/DNA film are also discussed.     

Copper chloride modified copper electrode: Application to electrocatalytic oxidation of methanol

Copper chloride modified copper (CCMC) electrode was prepared as a new electrode. For the preparation of the modified electrode, the polished copper electrode was placed in 0.1 M CuCl₂ solution for 20 s. In this step, a layer of copper (I) chloride was formed at the surface of copper electrode. Then, the electrode was placed in 0.1 M NaOH and the electrode potential was cycled between −250 and 1000 mV (vs. SCE) at a scan rate of 50 mV s⁻¹ for 5 cycles in a cyclic voltammetry regime until a featureless voltammogram was obtained. Surface physical characteristics of the modified electrode were studied by scanning electron micrographs (SEM). Results showed that considerable amounts of microcrystals have been formed on the copper surface during the modification. Surface elemental analysis of electrode were performed by energy dispersive X-ray (EDX) technique. The results showed that in addition to copper and chloride elements, there is also oxygen at the surface of CCMC electrode. This indicates that a layer of (ClCu)₂O was formed at the surface of the modified electrode. The electrocatalytic activity of the modified electrode for the oxidation of methanol, in aqueous basic solution was studied by using cyclic voltammetry. Results showed that, copper chloride modified electrode can improve the activity of Cu towards the oxidation of this small organic molecule, showing the possibility of attaining good electrocatalytic anodes for fuel cells. The modified electrode shows a stable and linear response in the concentration range of 5 × 10⁻³ to 8 × 10⁻² M with a correlation coefficient of 0.9958.     

Enhanced electrocatalytic activity for the oxidation of liquid fuels on PtSn nanoparticles

PtSn nanoparticles with different Pt/Sn ratio have been prepared by a chemical reduction method. XRD data indicate that Sn atom is introduced into the Pt lattice. Their electrocatalytic activity is evaluated using cyclic voltammetry (CV), differential electrochemistry mass spectrometry (DEMS), and rotating disk electrode (RDE) experiments. The Pt₉Sn₁ nanoparticles exhibit higher electrocatalytic activity than commercial Pt nanoparticles (E-TEK) for the oxidation of ethanol. The rate constants for the oxidation of formic acid, formaldehyde, methanol, ethanol, glycol, and glycerol on Pt₉Sn₁ electrocatalyst are much higher than that on Pt. The results indicate that Pt₉Sn₁ is an excellent electrocatalyst for the oxidation of liquid fuels. The activation energy studies show that the higher electrocatalytic activity of PtSn catalyst can be ascribed to the bifunctional mechanism, instead of the dilation of Pt crystal structure.     

Electrocatalytic activities of gold-5-amino-2-mercaptobenzimidazole-M self-assembled monolayer complexes (M: Ag, Cu) for hydroquinone oxidation investigated by CV and EIS

Electrocatalytic activity of a new catalyst toward the oxidation reaction of hydroquinone as a model compound is described. The catalyst was formed by immobilizing metal cations on the topside of a gold-5-amino-2-mercaptobenzimidazole, self-assembled monolayer (Au-5A2MBI-Mⁿ⁺ SAM, Mⁿ⁺: Cu²⁺, Ag⁺) electrode. Preparation steps and the electrocatalytic activity of the catalyst were studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The EIS data were approximated by appropriate electronic equivalent circuit models from which kinetic parameters, such as charge transfer resistance, double layer capacitance, and apparent rate constant (k_app), were estimated. Excellent activity was observed for Au-5A2MBI-Ag⁺ SAM with the following order: Au-5A2MBI-Ag⁺ > Au-5A2MBI-Cu²⁺ > Au-5A2MBI, after testing many modified electrodes. The increased activity originates from a modification of the Au-5A2MBI structure by mediating the effect of Ag⁺. This behavior was understood from significant increases in the k_app without significant changes in the double layer capacitance.