Electrocatalysis of NADH oxidation with an electropolymerized film of 1,4-bis(3,4-dihydroxyphenyl)-2,3-dimethylbutane

Synergistic effect of pyrroloquinoline quinone and graphene nano-interface for facile fabrication of sensitive NADH biosensor

A self-assembly composite of graphene-pyrroloquinoline quinone (PQQ) was fabricated and modified on glassy carbon electrode (GCE) for sensitive detection of nicotinamide adenine dinucleotide (NADH). Chitosan (CTS) was applied to disperse graphene to form a stable robust film on GCE. A synergistic effect between PQQ and graphene was observed during the electrocatalytic oxidation of NADH, with about 260mV reduction in the oxidation potential and 2.5-fold increase in the oxidation current compared with those on the bare GCE. The electrochemical sensors based on the modified electrodes allowed the detection of NADH with a good linear dependence from 0.32 to 220µM with a high sensitivity of 0.421µAµM^-1cm^-2 and a low detection limit of 0.16µM (S/N=3). It could also eliminate the interference of electroactive substances like ascorbic acid (AA), uric acid, and dopamine and its derivatives. The outstanding performances of graphene-PQQ/CTS composite capable of improving the electrical conductivity and accelerating the electron transport suggested its promising applications for design of different graphene based composites used in electrochemical sensing and energy fields.

Catechol-based biomimetic functional materials

Catechols are found in nature taking part in a remarkably broad scope of biochemical processes and functions. Though not exclusively, such versatility may be traced back to several properties uniquely found together in the o-dihydroxyaryl chemical function; namely, its ability to establish reversible equilibria at moderate redox potentials and pHs and to irreversibly cross-link through complex oxidation mechanisms; its excellent chelating properties, greatly exemplified by, but by no means exclusive, to the binding of Fe(3+); and the diverse modes of interaction of the vicinal hydroxyl groups with all kinds of surfaces of remarkably different chemical and physical nature. Thanks to this diversity, catechols can be found either as simple molecular systems, forming part of supramolacular structures, coordinated to different metal ions or as macromolecules mostly arising from polymerization mechanisms through covalent bonds. Such versatility has allowed catechols to participate in several natural processes and functions that range from the adhesive properties of marine organisms to the storage of some transition metal ions. As a result of such an astonishing range of functionalities, catechol-based systems have in recent years been subject to intense research, aimed at mimicking these natural systems in order to develop new functional materials and coatings. A comprehensive review of these studies is discussed in this paper.

Immobilized redox mediators for electrochemical NAD(P)+ regeneration

The applicability of dissolved redox mediators for NAD(P)(+) regeneration has been demonstrated several times. Nevertheless, the use of mediators in solutions for sensor applications is not a very convenient strategy since the analysis is not reagentless and long stabilization times occur. The most important drawbacks of dissolved mediators in biocatalytic applications are interferences during product purification, limited reusability of the mediators, and their cost-intensive elimination from wastewater. Therefore, the use of immobilized mediators has both economic and ecological advantages. This work critically reviews the current state-of-art of immobilized redox mediators for electrochemical NAD(P)(+) regeneration. Various surface modification techniques, such as adsorption polymerization and covalent linkage, as well as the corresponding NAD(P)(+) regeneration rates and the operational stability of the immobilized mediator films, will be discussed. By comparison with other existing regeneration systems, the technical potential and future perspectives of biocatalytic redox reactions based on electrochemically fed immobilized mediators will be assessed.

Electrocatalytic oxidation of NADH at electrogenerated NAD+ oxidation product immobilized onto multiwalled carbon nanotubes/ionic liquid nanocomposite: application to ethanol biosensing

The multiwalled carbon nanotubes/N-butyl-N-methyl-pyrolydinium-bis(trifluoromethylsulfonyl)imide [C(4)mpyr][NTf(2)] ionic liquid (MWCNTs/IL) modified glassy carbon (GC) electrode has been utilized as a platform to immobilize electrogenerated NAD(+) oxidation products (Ox-P(NAD(+))). During potential cycling, the adenine moiety of NAD(+) molecule is oxidized and gives rise to generation of a redox active system that shows great electrocatalytic activity toward NADH oxidation. The cyclic voltammetric results indicated the ability of MWCNTs/IL/Ox-P(NAD(+)) modified GC electrode to catalyze the oxidation of NADH at a very low potential (0.05 V vs. Ag/AgCl) and subsequently, a substantial decrease in the overpotential by about 600 mV compared with the bare GC electrode. This modified electrode thus allowed highly sensitive amperometric detection of NADH with a very low limit of detection (2 × 10(-8) mol L(-1)), low applied potential (+0.05 V) at concentration range up to 4.2 × 10(-5) mol L(-1) and minimum of surface fouling. High ability of MWCNTs/IL/Ox-P(NAD(+)) to promote electron transfer between NADH and the electrode suggested a new promising biocompatible platform for development of dehydrogenase-based amperometric biosensors. With alcohol dehydrogenase (ADH) as a model enzyme, ethanol sensing ability of the proposed system was examined. The amperometric response of the biosensor increased linearly with increasing ethanol concentration in two concentration ranges, 5 × 10(-6)-6 × 10(-5) and 6 × 10(-5)-9 × 10(-4) mol L(-1) with detection limit of 5 × 10(-7) mol L(-1) and rapid response of 10s. Furthermore, the interference effects of redox active species, such as ascorbic acid, uric acid, glucose and acetaminophen for the proposed biosensor are negligible. Finally, the ability of the proposed biosensor for detection of ethanol in real complex samples was successfully demonstrated.

Functionalized gold nanoparticles and films stabilized by in situ formed polyeugenol

Eugenol (2-methoxy-4-allyl-phenol) was used as a reducing agent for one-pot synthesis of gold nanoparticles in a mild alkaline aqueous/organic solution at room temperature. In this reaction, eugenol acts also as a stabilizing agent, since it undergoes polymerization upon oxidation. As a result, stable colloids of polyeugenol (PE)-capped gold nanoparticles are formed during the reaction with the average particle size of 44 nm. Moreover, conducting supports, such as ITO glass, are covered by Au/PE composite film when immersed in the reaction medium. The modified ITO shows redox activity assignable to residual quinone moieties of PE with redox couples at a potential range of -0.2 to 0.4V (vs. Ag/AgCl at pH 7.4). Redox properties of Au/PE nanoparticulate films can be exploited for the electrocatalytic oxidation of NADH with over 0.5 V reduction of the reaction overpotential vs. unmodified ITO. Nanoparticulate composite films were characterized by UV-vis spectroscopy, XPS and FT-IR spectroscopy. The characterization revealed structural similarity of the formed PE to lignin.

The application of novel spindle-like polypyrrole hollow nanocapsules containing Pt nanoparticles in electrocatalysis oxidation of nicotinamide adenine dinucleotide (NADH)

Novel spindle-like polypyrrole hollow nanocapsules containing Pt nanoparticles (Pt NPs/PPy composite hollow nanospindles) were successfully prepared by using beta-akaganeite (β-Fe(3+)O(OH,Cl)) nanospindles as templates and methanoic acid as a reducing agent. The β-Fe(3+)O(OH,Cl) templates can be easily obtained in ethanol/water mixing solution in the presence of thiophene and FeCl(3)·6H(2)O, and after coating by PPy shell, they can be gradually and completely etched during the reduction of H(2)PtCl(6) into Pt nanoparticles (Pt NPs) with the average size of 3.6 nm on spindle-like polypyrrole hollow nanocapsules, which could still keep their integrality of morphologies with the thickness of PPy shell of 18-20 nm. The investigation of Pt NPs/PPy composite hollow nanospindles modified glassy carbon electrode (GCE) for the application to detect nicotinamide adenine dinucleotide (NADH) with cyclic voltammetry (CV) and amperometry indicated good linearity and sensitivity of responses in the certain range of NADH concentration. The influence of Pt NPs content to the NADH oxidation current was also studied. This new kind of unique spindle-like noble metal/conducting polymer hollow nanostructured complex can be acted as a good steady electrode material for electrocatalytic oxidation of NADH.

6-Vinyl coenzyme Q0: Electropolymerization and electrocatalysis of NADH oxidation exploiting poly-p-quinone-modified electrode surfaces

6-Vinyl coenzyme Q(0) serves as a convenient starting material for the formation of electropolymerized coenzyme Q(0) on glassy carbon electrodes and the modified electrodes displays electrocatalytic activity toward NADH (β-nicotinamide adenine dinucleotide) oxidation. The detection of NADH was measured by differential pulse voltammetry, which reveals that the peak current is linear to the concentration of NADH within the range of 10-100μM. This would be helpful for the understanding of the interaction between coenzyme Q(0) and NADH in the biological process.

A novel ferroceneylazobenzene self-assembled monolayer on an ITO electrode: photochemical and electrochemical behaviors

A novel ferroceneylazobenzene self-assembled monolayer (SAM) has been constructed on an indium-tin oxide (ITO) electrode via the covalent attachment of 4-(4'-11-ferrocenyl-undecanoxyphenylazo)benzoic acid ( FcAzCOOH) onto a silanized ITO substrate surface and verified by reflectance infrared spectroscopy and water contact angle. Atomic force microscopy (AFM) and cyclic voltammogram (CV) indicated that the FcAzCOOH formed a uniform and reproducible SAM on the ITO electrode with a surface coverage of ca. 1.9 x 10 (-10) mol/cm (2) (87 A (2)/molecule). The reversible photoisomerization behavior of the SAM was characterized by UV-vis spectra. The azo pi-pi* transition band intensity of the SAM gradually decreased with UV (365 nm) irradiation and was almost recovered again when subsequent exposure to ambient room light (400-800 nm). The increased tilt angle of the molecules on the ITO substrate after UV irradiation further confirmed the trans-to- cis isomerization of azobenzene moieties. The CV of the trans- FcAzCOOH modified ITO electrode showed a pair of waves due to redox of the ferrocene groups in the potential range of 0 to +800 mV (vs SCE), and the peak separation of the redox wave became larger after UV irradiation and almost returned to its original value after subsequent exposure to the visible light. Rate-dependent CV curves indicated that the charge transfer rate between the ferrocene species in the SAM and the ITO electrode was slowed down after UV irradiation due to the smaller porosity of the monolayer film and the more compact barrier layer between the redox species and the ITO electrode. It is the first time to directly observe the influence of photoisomerization of the azobenzene moiety on the redox behavior of redox species in the ferroceneylazobenzene-functionalized SAM. The present results provide profound insight into the role of redox microenvironment on electron transfer kinetics and also provide a simple and facile approach to the preparation of photocontrollable electrodes.

Synthesis, characterization and application in electrocatalysis of polyaniline/Au composite nanotubes

Polyaniline (PANI) nanotubes were successfully synthesized using the hydrothermal method via an in situ polymerization. In the process, a fibrillar complex of FeCl(3) and methyl orange (MO), acting as the reactive self-degraded templates, directed the growth of PANI on its surface and promoted the assembly into nanotubular structures. By introducing PANI nanotubes into Au colloid, Au nanoparticles (NPs) could be decorated onto the PANI nanotube surface through the electrostatic effect. The morphology of the nanotubes and the number of decorated Au NPs could be controlled effectively by adjusting the experimental conditions. The resulting products were characterized by transmission electron microscopy (TEM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). In addition, the electrocatalytic activity of the composites towards the oxidation of NADH (nicotinamide adenine dinucleotide) was studied by immobilizing PANI/Au composites on the surface of a glassy carbon electrode (GCE).

Low-potential nicotinamide adenine dinucleotide detection at a glassy carbon electrode modified with toluidine blue O functionalized multiwall carbon nanotubes

The toluidine blue O (TBO) functionalized multiwall carbon nanotubes (MWNTs) nanomaterials (TBO-MWNTs) were prepared by assembling TBO onto the surface of a MWNTs modified glassy carbon (GC) electrode. Also TBO-MWNTs modified GC electrodes exhibiting a strong and stable electrocatalytic response toward beta-nicotinamide adenine dinucleotide (NADH) were described. Compared with a bare GC electrode, the TBO-MWNTs modified GC electrodes could decrease the oxidization overpotential of NADH by 730 mV, with a peak current at 0.0 V, since there was a positively synergistic electrocatalytic effect between the MWNTs and TBO toward NADH. Furthermore, the TBO-MWNTs modified GC electrodes had perfect performances, such as a low detection limit (down to 0.5 microM), being very stable (the current diminutions is lower than 6% in a period over 35 min), a fast response (within 3 s), and a wide linear range (from 2.0 microM to 3.5 mM). Such an ability of TBO-MWNTs to promote the NADH electron-transfer reaction suggests great promise for dehydrogenase-based amperometric biosensors.

5-Hydroxytryptophan as a precursor of a catalyst for the oxidation of NADH

Following oxidation of 5-hydroxytryptophan (5-HTPP) at a pyrolytic graphite electrode at pH 7.5, two quasi-reversible redox couples emerge at -0.170 and +0.032 V, respectively, due to oxidation products strongly adsorbed to the electrode surface. These redox processes have been electrochemically and kinetically characterized in terms of the dependence of the formal potential (E degrees ') with pH, variation of the current density with scan rate, operational stability, and electron-transfer rate constant (k(s)). The wave centered at +0.032 V could mediate the oxidation of NADH, exhibiting a strong and persistent electrocatalytic response. A quinone-imine structure has been proposed as the electrocatalytically active species. The kinetics of the reaction between the mediator and NADH has been characterized via rotating disk electrode voltammetry, and it has been found that the rate constant for the reaction is dependent on the solution concentration of NADH. 5-HTPP modified electrodes could be employed in the amperometric detection of NADH with a limit of detection in the nanomolar range. Moreover, 5-HTPP modified electrodes retain their electrocatalytic activity for at least one week. The potential application of these electrodes to amperometric biosensor is demonstrated.

Electrochemically triggered Michael addition on the self-assembly of 4-thiouracil: generation of surface-confined redox mediator and electrocatalysis

Generation of a surface-confined redox mediator (RM) by an electrochemically triggered Michael addition reaction and the electrocatalytic properties of the mediator are described. Electrogenerated o-quinone undergoes Michael addition reaction with the self-assembled monolayer (SAM) of 4-thiouracil (4-TU) on a gold (Au) electrode and yields a surface-confined RM, 1-(3,4-dihydroxyphenyl)-4-mercapto-1H-pyrimidin-2-one (DPTU). The Michael addition reaction depends on the electrolysis potential and time, solution pH, and concentration of catechol (CA) used in the reaction. The redox mediator, DPTU, exhibits reversible redox response, characterstic of a surface-confined species at approximately 0.22 V in neutral pH. The anodic peak potential of DPTU shifts by 58+/-2 mV while changing the solution pH by one unit, suggesting that protons and electrons taking part in the redox reaction are in the ratio of 1:1. The apparent rate constant (ksapp) for the heterogeneous electron-transfer reaction of the RM was determined to be 114+/-5 s-1. The surface coverage (Gamma) of DPTU on the electrode surface was 8.2+/-0.1x10(-12) mol/cm2. DPTU shows excellent electrocatalytic activity toward oxidation of reduced nicotinamide adenine dinucleotide (NADH) with activation overpotential, which is approximately 600 mV lower than that observed at the unmodified Au electrode. The dipositive cations in the supporting electrolyte solution amplify the electrocatalytic activity of DPTU. A 2.5-fold enhancement in the catalytic current was observed in the presence of Ca2+ or Ba2+ ions. The sensitivity of the electrode toward NADH in the presence and absence of Ca2+ ions was 0.094+/-0.011 and 0.04+/-0.0071 nA cm-2 nM-1, respectively. A linear increase in the catalytic current was obtained up to the concentration of 0.8 mM, and the electrode can detect amperometrically as low as 25 nM of NADH in neutral pH.

Electropolymerized flavin adenine dinucleotide as an advanced NADH transducer

Electropolymerizing the prosthetic group (flavin adenine dinucleotide, FAD) responsible in the active sites of dehydrogenases for NAD(+)|NADH regeneration, we succeeded in mimicking enzyme activity. Poly(FAD) characterized by an additional polymer-type redox reaction has been discovered as a highly effective electrocatalyst for NADH oxidation: operating at the lowest potentials reported for NADH transducers (0.00 V, pH 7.4), poly(FAD) is characterized by the electrochemical rate constant of 1.8 +/- 0.6 x 10(-3) cm s(-1), which is at the level of the NADH mass-transfer constant. Flow injection analysis of NADH with the poly(FAD)-modified wall-jet electrode as a detector has been characterized by a linear calibration range prolonged down to 5 x 10(-7) M and a sensitivity of 0.08 A M(-1) cm(-2), which taking into account the dispersion coefficient ( approximately 3), is at the diffusion-limiting value. In contrast to the low molecular weight mediators able to exhibit similar electrocatalytic properties, poly(FAD)-modified electrodes are characterized by the dramatically improved stability and, thus, can be considered as the most advantageous NADH transducers for analytical chemistry.

Mediator-modified electrodes for catalytic NADH oxidation: high rate constants at interesting overpotentials

Carbon paste electrodes were modified with a nitrofluorenone derivative, 2,4,7-trinitro-9-fluorenone, adsorbed on zirconium phosphate (ZP). After electrochemical reduction of the fluorenone derivative, it turns into a very efficient mediator for electrocatalytic NADH oxidation, with a formal potential of about +250 mV vs. Ag/AgCl. The electrochemistry and the electrocatalytic properties of the mediator were investigated with cyclic voltammetry and rotating disk electrode methodology. The second order rate constant with NADH was evaluated and found to be higher than 10(6) M(-1) s(-1), thus approaching true diffusion controlled currents for NADH oxidation.