Katalytische Oxidation von NADH an Graphitelektroden durch adsorbierte Phenoxazinderivate

Flow-Injection Analysis and Voltammetric Detection of NADH with a Poly-Toluidine Blue Modified Electrode

Poly-Toluidine Blue film was prepared by electrooxidative polymerization at a glassy carbon electrode in a phosphate buffer solution. The resulting chemically modified electrode (CME) exhibited excellent electrocatalysis toward the oxidation of reduced nicotinamide coenzyme (NADH) with over a 450 mV decrease of the overpotential compared with that at a bare glassy carbon electrode. Two electrochemical determinations of NADH, cyclic voltammetry and flow injection analysis, were established based on the electrocatalytical performance of the resulting modified electrode. Under an identical determinate condition, the voltammetric detection for NADH gave a detection limit of 3.3 micromol L(-1) with a linear concentration range of 9.1 micromol L(-1) to 1.8 mmol L(-1). As a detector in a flow-injection system, the CME gave a detection limit of 0.1 micromol L(-1) for NADH with a linear concentration range of 1.0 micromol L(-1) to 3.2 mmol L(-1). Obviously, flow-injection analysis is superior to voltammetric detection in NADH determination for its lower detection limit and wider detectable linear range.

Hydrogen peroxide sensor based on coimmobilized methylene green and horseradish peroxidase in the same montmorillonite-modified bovine serum albumin-glutaraldehyde matrix on a glassy carbon electrode surface

A new approach to construct a second-generation amperometric biosensor is described. The classical dye methylene green as a probing-needle mediator and horseradish peroxidase as a base enzyme were coimmobilized in the same montmorillonite-modified bovine serum albumin (BSA)-glutaraldehyde matrix to construct a H2O2 sensor. The immobilization matrix was formed from the pretreated sodium montmorillonite colloid in which the enzyme and the cross-linker were dissolved. Immobilization of methylene green from the dye mother solution was attributed to the adsorption function of the montmorillonite, whereas immobilization of horseradish peroxidase was attributed to the cross-linking function of the BSA-glutaraldehyde as usual. Cyclic voltammetry and potentiostatic measurements indicated that methylene green efficiently mediated electrons from the base electrode to the enzyme in the matrix. The sensor responded rapidly to low H2O2 concentration and achieved 95% of the steady-state current in less than 20 s, with a detection limit of 4.0 x 10(-7) M H2O2.

Poly(methylene blue)-modified thick-film gold electrodes for the electrocatalytic oxidation of NADH and their application in glucose biosensors

Electropolymerization of the phenothiazine derivative methylene blue (MB) on screen-printed, thick-film gold electrodes leads to electrocatalytically active and conducting layers of poly(methylene blue) (PMB) in intimate and stable contact with the electrode surface. The catalytic properties of the PMB films allow anodic oxidation of NADH at potentials as low as +200 mV vs. the saturated calomel electrode (SCE) reducing interferences from cooxidizable species as well as minimizing electrode fouling by enabling a simultaneous two-electron transfer mechanism. Dehydrogenase-based biosensors employing PMB-modified thick-film electrodes are obtained either by entrapment of the enzyme into the PMB layer itself or by laminating an enzyme membrane made of an aqueous poly(vinylacetate) dispersion over the PMB-modified electrode. Both methods are used to fabricate glucose biosensors which can be operated at low overpotentials, i.e. +200 mV vs. SCE.

Amperometric thin film biosensors based on glucose dehydrogenase and Toluidine Blue O as catalyst for NADH electrooxidation

Amperometric glucose sensors were constructed based on solid graphite electrodes, surface-modified with NAD+ dependent glucose dehydrogenase (GDH), Toluidine Blue O (TBO), and protective ionic polymers. The electrocatalytic oxidation of NADH was evaluated from cyclic voltammetry with TBO dissolved, adsorbed, and electrostatically or covalently bound to polymers. The NADH and glucose sensors constructed were investigated and operated at 0 mV vs. Ag/AgCl using single potential step chronoamperometry. The operational stability of the glucose sensors was limited by leakage of NAD+. A glucose sensitivity much higher than carbon paste electrode was found. A sensitivity as high as 25 microA cm-2 mM-1 was achieved.

[Biosensors]

By the combination of transducers (thermistors, selective electrodes, field-effect transistors, optical systems) with immobilized enzymes or antibodies specific sensors for biologically relevant substances are obtained. The construction, ranges, of linearity, response times and stability of biosensors are demonstrated. Examples are given for their application in clinical analysis and fermentation control. Finally limits and future possibilities are discussed.