Electrodes modified with nitrofluorenone derivatives as a basis for new biosensors

Electrocatalysis of NADH oxidation using electrochemically activated fluphenazine on carbon nanotube electrode

Electrocatalytic determination of NADH using a hybrid surface-modified electrode with multi-wall carbon nanotubes (MWCNTs) and a novel electrogenerated redox mediator is described. The redox mediator precursor - fluphenazine (Flu) was adsorbed on MWCNT-modified glassy carbon (GC) electrode which was then subjected to electrochemical activation in 0.1 M H2SO4 using cyclic voltammetry (CV) over a range of potentials -0.2 to 1.5 V vs. Ag/AgCl (6 scans at 100 mV s(-1)). Cyclic voltammograms of Flu indicated the formation of a stable electroactive material presenting one reversible redox couple at the formal potential of -0.115 vs. Ag/AgCl in a phosphate buffer (pH7.0) as a supporting electrolyte. The peaks increased linearly with increasing scan rate indicating electroactive molecules anchored to the electrode surface. The GC/MWCNT/Flu electrode efficiently catalyzed the oxidation of NADH with a decrease in the overpotential of about 600 mV and 150 mV compared to the bare GC and GC/MWCNT electrode, respectively. This modified electrode was successfully used as the working electrode in the chronoamperometric analysis. The peak current response to NADH was linear over its concentration range from 15 μM to 84 μM, and correlation coefficient 0.998. The limits of detection (5 μM) and quantitation (15 μM) were evaluated.

THE APPLICATION OF SELF-ASSEMBLY MONOLAYER TECHNIQUE INTO FABRICATING MULTILAYER NANOFILM

Self-Assembly Monolayer (SAM) technique, as a novel and developing technique for fabricating layer-by-layer nanofilm on substrates of various sizes, shapes and materials, has received more and more attention in the areas of light-emitting devices, nonlinear optical materials, conductive films, permselective gas membranes, sensors, modification of electrodes, resistance and printing technique. In comparison with other traditional methods, SAM technique has many significant advantages, including simple process, universality, formation with densely packed, well defined, highly ordered surfaces. This paper will give a review on the recent development in SAM technique.

Multiscale-tailored bioelectrode surfaces for optimized catalytic conversion efficiency

We describe the elaboration of a multiscale-tailored bioelectrocatalytic system. The combination of two enzymes, D-sorbitol dehydrogenase and diaphorase, is studied with respect to the oxidation of D-sorbitol as a model system. The biomolecules are immobilized in an electrodeposited paint (EDP) layer. Reproducible and efficient catalysis of D-sorbitol oxidation is recorded when this system is immobilized on a gold electrode modified by a self-assembled monolayer of 4-carboxy-(2,5,7-trinitro-9-fluorenylidene)malonitrile used as a mediator. The insertion of mediator-modified gold nanoparticles into the EDP film increases significantly the active surface area for the catalytic reaction, which can be further enhanced when the whole system is immobilized in macroporous gold electrodes. This multiscale architecture finally leads to a catalytic device with optimized efficiency for potential use in biosensors, bioelectrosynthesis, and biofuel cells.

Facilitation of NADH electro-oxidation at treated carbon nanotubes

The relationship between the state of the surface of carbon nanotubes (CNTs) and their electrochemical activity was investigated using the enzyme cofactor dihydronicotinamide adenine dinucleotide (NADH) as a redox probe. The boiling of CNTs in water, while nondestructive, activated them toward the oxidation of NADH, as indicated by a shift in the anodic peak potential of NADH (E(NADH)) from 0.4 V to 0.0 V. The shift in E(NADH) was due to the redox mediation of NADH oxidation by traces of quinone species that were formed on the surface of treated CNTs. The harsher treatment that was comprised of microwaving CNTs in concentrated nitric acid had a similar effect on the E(NADH), and, additionally, it increased the anodic peak current of NADH. The latter correlated with the formation of defects on the surface of acid-microwaved CNTs, as indicated by their Raman spectra. The increase in current was discussed, considering the role of surface mediators on the buckled graphene sheets of acid-microwaved CNTs. The other carbon allotropes, including the edge-plane pyrolytic graphite, graphite powder, and glassy carbon, did not display a comparable activation toward the oxidation of NADH.

Poly(brilliant cresyl blue)-carbonnanotube modified electrodes for determination of NADH and fabrication of ethanol dehydrogenase-based biosensor

A single walled-carbon nanotube (SWNT) modified with poly brilliant cresyl blue (PBCB) glassy carbon electrode has been fabricated by a simple, method in order to facilitate electrocatalytic detection of NADH. At this chemically modified electrode, NADH was determined in neutral phosphate buffer solution at 0V (vs. SCE). The amperometric detection provided a wide linear current vs. concentration range (3.0-104.2 microM), a fast response time (within 5s), high sensitivity [9.89nA (muM)(-1)] and a low detection limit (1.0 microM, S/N=3). No interference was observed with a 100-fold excess of dopamine or uric acid. An ethanol biosensor also was developed using the nanocomposite modified electrode, by immobilizing ethanol dehydrogenase with carrageenan. In this case a linear ethanol concentration response was achieved in the range from 0.4 to 2.4mM and the detection limit was estimated to be 0.1mM (S/N=3). The analytical performance achieved with the of the PBCB/SWNT nanocomposite electrode is expected to the development of novel biosensors, biofuel cells, and other bioelectrochemical devices.

Tryptophan repressor-binding proteins from Escherichia coli and Archaeoglobus fulgidus as new catalysts for 1,4-dihydronicotinamide adenine dinucleotide-dependent amperometric biosensors and biofuel cells

The tryptophan (W) repressor-binding proteins (WrbA) from Escherichia coli (EcWrbA) and Archaeoglobus fulgidus (AfWrbA) were investigated for possible use in 1,4-dihydronicotinamide adenine dinucleotide (NADH) dependent amperometric biosensors and biofuel cells. EcWrbA and AfWrbA are oligomeric flavoproteins binding one flavin mononucleotide (FMN) per monomer and belonging to a new family of NAD(P)H:quinone oxidoreductases (NQOs). The enzymes were covalently linked to a low potential Os redox polymer onto graphite in the presence of single-walled carbon nanotube (SWCNT) preparations of varying average lengths. The performance of the enzyme modified electrodes for NADH oxidation was strongly depending on the average length of the applied SWCNTs. By blending the Os redox polymer with SWCNTs, the electrocatalytic current could be increased up to a factor of 5. Results obtained for AfWrbA modified electrodes were better than those for EcWrbA. For NADH detection, a linear range between 5 microM and 1 mM, a lower limit of detection of 3 microM, and a sensitivity of 56.5 nA microM(-1) cm(-2) could be reached. Additionally spectroelectrochemical measurements were carried out in order to determine the midpoint potentials of the enzymes (-115 mV vs NHE for EcWrbA and -100 mV vs NHE for AfWrbA pH 7.0). Furthermore, an AfWrbA modified electrode was used as an anode in combination with a Pt black cathode as a biofuel cell prototype.

Increasing amperometric biosensor sensitivity by length fractionated single-walled carbon nanotubes

In this work the sensitivity-increasing effect of single-walled carbon nanotubes (SWCNTs) in amperometric biosensors, depending on their average length distribution, was studied. For this purpose the SWCNTs were oxidatively shortened and subsequently length separated by size exclusion chromatography. Transmission electron micrographs of different fractions of SWCNTs were collected. Diaphorase "wired" to an osmium redox polymer was blended with the shortened SWCNTs of different lengths. Depending on the average length of the SWCNTs the sensitivity of the amperometric biosensor model system towards oxidation of 1,4-dihydronicotinamide adenine dinucleotide (NADH) was increased by a factor of five. The best performance was achieved with SWCNTs of medium length. The linear range for NADH detection was between 5 microM and 7 mM, the maximum sensitivity was 47 nA microM(-1) cm(-2), and the detection limit was 1 microM. The biosensor exhibited excellent electrocatalytic properties. Even at relatively high NADH concentrations the oxidative current was limited by the diffusion rate of NADH.

Optimized carbon nanotube fiber microelectrodes as potential analytical tools

The preparation and interesting electrochemical properties of carbon nanotube (CNT) fiber microelectrodes are reported. By combining the advantages of CNT with those of fiber electrodes, this type of microelectrode differs from CNT-modified or CNT-containing composite electrodes, because they are made solely of CNT without other components, for example additives or binders. The performance of these electrodes has been characterized with regard to, among others, the electrocatalytic oxidation of analytes via dehydrogenase-mediated reactions. In this context the reversible regeneration of the coenzyme NAD(+) using a mediator is a key step in the development of new amperometric sensor devices and we have successfully immobilized mediator molecules that are very efficient for this purpose on the surface of the CNT fiber electrode. The microelectrodes thus obtained have been compared with classic carbon microelectrodes and have promising behavior in biosensing applications, especially after specific pretreatments such as CNT alignment inside the fiber or expansion of the specific surface by chemically induced swelling.

Development of a commercial amperometric biosensor electrode for the ketone D-3-hydroxybutyrate

Representatives of the common classes of quinoid NADH redox mediator, including Meldola Blue (MB) 3, 4-methyl-1,2-benzoquinone (4-MBQ) 4, 1-methoxy phenazine methosulphate (1-MeO-PMS) 5 and 2,6-dichloroindophenol (DCIP) 6, are shown to inhibit the NAD-dependent enzyme D-3-hydroxybutyrate dehydrogenase (HBDH), severely limiting their utility in the construction of a stable biosensor electrode for the ketone body D-3-hydroxybutyrate (3-OHB). It is proposed that these mediators bind covalently to important thiol groups in the enzyme. This mode of inhibition is overcome through the use of mediators such as 1,10-phenanthroline quinone (1,10-PQ) 7, which avoid 1,4-nucleophilic addition with enzyme amino acid residues such as Cys. As a result, 1,10-PQ 7 was selected for incorporation in a biosensor electrode for 3-OHB. The resulting MediSense Optiumtrade mark beta-Ketone electrode is stable (<or=10% loss in response at 30 degrees C versus 4 degrees C) with a long shelf life of 18 months. Diabetics can determine their D-3-hydroxybutyrate level with good precision (0.43 mM 3-OHB, 10.5% CV; 1.08 mM, 5.9%; 3.55 mM, 3.2%; n=20 per level) and accuracy (versus reference assay: slope=0.98; intercept=0.02 mM, r=0.97, n=120) over the range 0.0-6.0 mM in 30 s using a small volume of blood (5 microl). The electrode has a low operating potential (+200 mV versus Ag/AgCl) such that the effect of electroactive agents in blood is minimised.

A comparison between the use of a redox mediator in solution and of surface modified electrodes in the electrocatalytic oxidation of nicotinamide adenine dinucleotide

Cyclic voltammetry was successfully applied to study the oxidation of nicotinamide adenine dinucleotide (NADH) both in homogeneous and heterogeneous phase. The first case was realized with a solution containing p-methylamino-phenolsulphate (MAP) as redox mediator and the diaphorase (DI) from Clostridium kluveri as enzyme while the second one by using both a glassy carbon (GC) and a carbon nanotube paste (CNTP) electrode modified with electrodeposited films derived from 3,4-dihydroxybenzaldehyde (3,4-DHB). Such systems were successively coupled with glucose dehydrogenase (GDH) reaction to realize the redox chain present in glucose biosensors. A critical comparison of the two systems was also reported.

Electrocatalytic oxidation of NADH at single-wall carbon-nanotube-paste electrodes: kinetic considerations for use of a redox mediator in solution and dissolved in the paste

Cyclic voltammetry has been successfully used to study the oxidation of nicotinamide adenine dinucleotide (NADH) at single-wall carbon-nanotube-paste (CNTP) electrodes modified with p-methylaminophenolsulfate (p-MAP) and 3,4-dihydroxybenzaldehyde (3,4-DHB). Diffusion-like behaviour was observed for p-MAP-modified electrodes, and a diffusion coefficient of 2.4x10(-6) cm2 s(-1) was calculated for p-MAP in the paste. The behaviour of 3,4-DHB-modified CNTP electrodes was typical of that of surface-confined mediators. p-MAP electrocatalytic activity was first checked in solution, and a rate constant of 9.2 mol(-1) L s(-1) was obtained for the reaction between NADH and the mediator. The p-MAP-modified electrode did not have significant electrocatalytic activity for electro-oxidation of NADH, probably because of the formation of a complex between NADH and the confined mediator. In contrast, the 3,4-DHB-modified electrode had very good NADH electrocatalytic activity, with a heterogeneous rate constant of approximately 20x10(2) mol(-1) L s(-1).

Electrochemical oxidation of biological molecules at carbon paste electrodes pre-treated in guanine solutions

Carbon paste electrodes were modified in guanine solutions under an applied potential of 1.1 V and used for electrochemical detection of NADH, NADPH, uric acid and 8-oxoguanine. Detection limits were 3.3, 3.7, 6.6 and 2.0 x 10(-6) M, respectively, with sensitivity of 0.13, 0.10, 0.26 and 0.40 A mol(-1) l cm(-2), respectively. The electrodes showed high reproducibility and absence of surface poisoning effects. Good analytical performance was attributed to the formation of superficial dimer or trimers species of guanine during the modification process.

Affinity assembled multilayers for new dehydrogenase biosensors

We propose a novel approach, which allows the control of the spatial arrangement of redox mediator, coenzyme and enzyme on the electrode at a molecular level, using essentially electrostatic interactions. The first step consists of adsorbing a monolayer of molecules out of a new family of redox mediators, substituted nitrofluorenones. In a second step, a monolayer of calcium cations is immobilized at the interface. It serves as a bridge between the redox mediator and the subsequently adsorbed coenzyme. The weak interaction between a carboxyl group of the redox mediator and the coenzyme's phosphate groups, revealed by QCM measurements, allows the coenzyme to keep its natural activity in the adsorbed state. In the last step, we use the intrinsic affinity of this monolayer of NAD(+) for dehydrogenases to build up a supramolecular sandwich composed of mediator/Ca(2+)/NAD(+)/dehydrogenase. This simple modification procedure, which might constitute a versatile approach for the low cost assembly of well-defined biosensors surfaces, has been successfully applied to the enzymatic detection of glucose, glutamate and alcohol.