Flavin adenine dinucleotide as precursor for NADH electrocatalyst

The generation of a new electrocatalytic system for NADH after oxidizing flavin adenine dinucleotide (FAD) is shown. The oxidation is performed in alkaline medium until +1.4 V (Ag/AgCl) at graphite electrodes. The catalytic activity is ascribed to the electrooxidized moiety of FAD and not to quinone surface groups. A comparison between this catalyst and that attributed to poly(FAD) (Karyakin, A. A.; Ivanova Y. N.; Revunova, K. V.; Karyakina, E. E. Anal. Chem. 2004, 76, 2004-2009.) is presented. It is concluded that the surface quinone groups generated during the strong anodization of the electrode in acidic medium at 2-2.5 V and not the poly(FAD) are responsible for the catalytic activity described in the above mentioned work.     

Electron-transfer studies with a new flavin adenine dinucleotide dependent glucose dehydrogenase and osmium polymers of different redox potentials

A new extracellular flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase from Glomerella cingulata (GcGDH) was electrochemically studied as a recognition element in glucose biosensors. The redox enzyme was recombinantly produced in Pichia pastoris and homogeneously purified, and its glucose-oxidizing properties on spectrographic graphite electrodes were investigated. Six different Os polymers, the redox potentials of which ranged in a broad potential window between +15 and +489 mV versus the normal hydrogen electrode (NHE), were used to immobilize and "wire" GcGDH to the spectrographic graphite electrode's surface. The GcGDH/Os polymer modified electrodes were evaluated by chronoamperometry using flow injection analysis. The current response was investigated using a stepwisely increased applied potential. It was observed that the ratio of GcGDH/Os polymer and the overall loading of the enzyme electrode significantly affect the performance of the enzyme electrode for glucose oxidation. The best-suited Os polymer [Os(4,4'-dimethyl-2,2'-bipyridine)(2)(PVI)Cl](+) had a potential of +309 mV versus NHE, and the optimum GcGDH/Os polymer ratio was 1:2 yielding a maximum current density of 493 μA·cm(-2) at a 30 mM glucose concentration.     

Electrochemical synthesis and characterization of TiO(2) nanoparticles and their use as a platform for flavin adenine dinucleotide immobilization and efficient electrocatalysis

Here, we report the electrochemical synthesis of TiO(2) nanoparticles (NPs) using the potentiostat method. Synthesized particles have been characterized by using x-ray diffraction (XRD) studies, atomic force microscopy (AFM) and scanning electron microscopy (SEM). The results revealed that the TiO(2) film produced was mainly composed of rutile and that the particles are of a size in the range of 100 ± 50?nm. TiO(2) NPs were used for the modification of a screen printed carbon electrode (SPE). The resulting TiO(2) film coated SPE was used to immobilize flavin adenine dinucleotide (FAD). The flavin enzyme firmly attached onto the metal oxide surface and this modified electrode showed promising electrocatalytic activities towards the reduction of hydrogen peroxide (H(2)O(2)) in physiological conditions. The electrochemistry of FAD confined in the oxide film was investigated. The immobilized FAD displayed a pair of redox peaks with a formal potential of -0.42?V in pH?7.0 oxygen-free phosphate buffers at a scan rate of 50?mV?s(-1). The FAD in the nanostructured TiO(2) film retained its bioactivity and exhibited excellent electrocatalytic response to the reduction of H(2)O(2), based on which a mediated biosensor for H(2)O(2) was achieved. The linear range for the determination of H(2)O(2) was from 0.15 × 10(-6) to 3.0 × 10(-3)?M with the detection limit of 0.1 × 10(-6)?M at a signal-to-noise ratio of 3. The stability and repeatability of the biosensor is also discussed.     

Processible polyaniline as an advanced potentiometric pH transducer. Application to biosensors.

An advanced potentiometric pH transducer based on processible polyaniline (PCPAn) is reported on. Both glassy carbon and screen-printed carbon electrodes modified with PCPAn by dip-coating exhibited a fully reversible potentiometric response of approximately 90 mV/pH unit over the range from pH 3 to 9. Such a significantly higher potentiometric response of PCPAn-modified electrodes as compared with those of existing devices is explained on the basis of the thermodynamics of polyaniline redox reactions. The PCPAn-based pH transducers exhibit both good operational stability and prolonged shelf life and display a negligible response toward singly charged cations. The new thick-film pH transducer was employed for designing a potentiometric biosensor for urea. In the model solution which mimics blood serum, the urea-sensitive electrode has a detection limit of 10(-5) M urea and a maximum response of approximately 120 mV. The attractive performance characteristics are advantageous over those of existing pH sensors and offer great promise for sensing and biosensing applications.     

Hall transducer as an UHF power indicator

Conclusions Hall transducers made of n-type germanium can be used as sufficiently efficient power indicators in the long-wave portion of the UHF range for medium power. A wattmeter of that type using a Hall transducer described above in conjunction with high quality resonators has a linear scale and a sensitivity of 1 V per 4 m W. The error in measuring power evaluated by deviations from the mean does not exceed ±5%.     

Thermostable reduced nicotinamide adenine dinucleotide oxidase: application to amperometric enzyme assay

The use in amperometric enzyme assays of a highly stable, pH insensitive flavoenzyme, reduced nicotinamide adenine dinucleotide oxidase (NADH oxidase), from the thermophilic organism Thermus aquaticus is described. The enzyme catalyses the oxidation of reduced nicotinamide adenine dinucleotide with concomitant two-electron reduction of dioxygen to hydrogen peroxide. In addition the enzyme used a substituted ferrocene as an alternative mediator of electron transfer. Hydrogen peroxide was detected at +650 mV vs Ag/AgCl at a platinum electrode. The current produced by oxidation of hydrogen peroxide was directly proportional to NADH concentration. The enzyme was used in solution to reoxidize enzymatically generated NADH and served as a basis for amperometric enzyme amplification systems for immunoassay as well as for the detection of substrate concentration for oxidoreductase enzymes. In the presence of alcohol dehydrogenase a rapid production of current occurred upon addition of ethanol over a clinically significant range. Thermus aquaticus NADH oxidase appears to be ideally suited for future exploitation in amperometric sensors for oxidoreductase substrates, offering a number of advantages over previously reported methods.     

Dual-enzyme fiber-optic biosensor for glutamate based on reduced nicotinamide adenine dinucleotide luminescence.

Response characteristics are presented for a dual-enzyme fiber-optic biosensor for glutamate. An enzyme layer composed of glutamate dehydrogenase (GDH) and glutamate-pyruvate transaminase (GPT) is used to produce reduced nicotinamide adenine dinucleotide (NADH) at the tip of a fiber-optic probe. NADH luminescence is monitored through this probe and the measured fluorescence intensity is related to the concentration of glutamate. GDH catalyzes the formation of NADH, and GPT drives the GDH reaction by removing a reaction product and regenerating glutamate. Optimal response is obtained in a pH 7.4 Tris-HCl buffer maintained at 25 degrees C in the presence of 4 mM NAD+ and 10 mM L-alanine. The temperature profile reveals a strong negative temperature effect which is attributed to the temperature dependency of NADH luminescence. Under optimal conditions, the sensor sensitivity is 0.127 nA/microM over the 1-10 microM concentration range, the detection limit is 0.13 microM, and response times range from 4 to 8 min. The sensor response is stable for 12 days when stored at 4 degrees C. Selectivity for glutamate is excellent over most of the common amino acids as well as ascorbic acid, uric acid, taurine, and GABA. Only slight responses were observed for glutamine and lysine. The effect of ammonia on the glutamate response was found to be minimal at total ammonia nitrogen concentrations as high as 200 microM.     

Use of the electrochromic behaviour of lanthanide phthalocyanine films for nicotinamide adenine dinucleotide detection

Electrochromic properties of spun films of bis[octakis(hexylthio)phthalocyaninato] dysprosium(III) were investigated for determining nicotinamide adenine dinucleotide hydride (NADH) in water solutions. A spin-coated film deposited on indium tin oxide electrode displays only one redox couple (at E1/2=0.78V). The films of [(C6H13S)8Pc]2Dy were modified chemically or electrochemically for the detection of reduced NADH in water solution. The modified film in the oxidized ([(C6H13S)8Pc]2Dy)+ form is believed to be reduced to its neutral form on interaction with NADH.     

The use of metallic glass scroll as an acoustic transducer element

A study was made of the piezomagnetic and elastic properties of scrolls of Metglas 2605 SC, used as an acoustic transducer element. This study involved quasi-DC magnetic induction measurements and ultrasonic complex plane analysis of scrolls of various annealing histories. An optimum effective magnetomechanical coupling coefficient. K(eff), of 0.75, was found. The best magnetomechanical response in air obtained for metallic glass scroll during the study was for the unconsolidated case. However, preliminary experiments have shown that structural weakness and corrosion problems are significant when using the unconsolidated scrolls in water.     

Focusing with an optoacoustic transducer

A prototype of an ultrasonic transducer has been developed that uses an optical input to control the ultrasonic output. This transducer is called an optoacoustic transducer (OAT) and provides an ultrasonic pattern that is spatially similar to the optical pattern used to illuminate it. When a focus-inducing optical pattern, such as a zone plate, is used, an acoustic focus is achieved. The success of this procedure depends on the use of amplitude-modulated light at the input and on filtering the received signal to eliminate the primary frequency. This provides an increase, from 30 dB to 70 dB, in the ratio of acoustic pressure in the illuminated regions to that in the dark regions. The prototype operates at 2.8 MHz and has been used to provide a good acoustic focus in water. A 3-dB beamwidth of 3.5 mm was measured at a range of 92 mm. The construction techniques and materials used are discussed.     

Superstable advanced hydrogen peroxide transducer based on transition metal hexacyanoferrates

We report on a superstable hydrogen peroxide (H(2)O(2)) transducer made by sequential deposition of the iron- and nickel-hexacyanoferrate (NiHCF) layers. Both chemical and mechanical stability of the latter, as well as similarity of its structure to Prussian Blue (PB) provide a substantial stabilization of the most advantageous H(2)O(2) transducer. The electrochemically deposited five bilayers of PB-NiHCF exhibit a complete stability under the continuous wall-jet flow of 1 mM of H(2)O(2) during more than 2 h, maintaining current at a level of 0.2 mA cm(-2), whereas common Prussian Blue loses half of its response within the first 20-25 min. Even being deposited in the open circuit regime on screen-printed electrodes, PB-NiHCF bilayers dramatically improve tolerance of the resulting transducer to alkaline solutions and iron ligands. Despite their 2-2.5 times decreased sensitivity (compared to common Prussian Blue), the sequentially deposited bilayers of PB-NiHCF provide a similar dynamic range of the transducer due to the decreased noise level.