Highly sensitive nitrite sensors have been developed for the first time based on mediator-modified electrodes. Tetraheme cytochrome c nitrite reductase from Sulfurospirillum deleyianum and cytochrome cd(1) nitrite reductase from Paracoccus denitrificans are able to accept electrons from artificial electron donors, which simultaneously act as electron mediators between the enzyme and an amperometric electrode. In addition to methyl viologen, redox-active compounds such as phenazines (phenosafranin, safranin T, N-methylphenazinium, 1-methoxy-N-methylphenazinium) and triarylmethane redox dyes (bromphenol blue and red) were selected from a range of redox compounds exhibiting the most efficient performance for nitrite detection. After precipitation, the electron mediators were incorporated in a graphite electrode material. Enzyme immobilization is performed by entrapment in a poly(carbamoyl sulfonate) (PCS) hydrogel. Diffusion coefficients and apparent heterogeneous rate constants of the mediators as well as homogeneous rate constants of nitrite sensors were determined by chronoamperometry and cyclic voltammetry. The phenosafranin-modified electrode layered with the PCS hydrogel immobilization of tetraheme cytochrome c nitrite reductase yielded linear current responses up to 250 μM nitrite with a sensitivity of 446.5 mA M(-)(1) cm(-)(2). The detection limit of the enzymatic nitrite sensor was found to be 1 μM nitrite.
A methylphenazonium-zeolite-modified enzyme sensor based on a planar, screen-printed, two-electrode arrangement is described for the subnanomolar detection of phenols. Using tyrosinase (EC 1.14.18.1), a novel polyurethane hydrogel was applied for immobilization and stabilization of the enzyme, which forms a self-adhering layer on the active surface of the strip sensor. Performance and characteristics of the sensor were evaluated with regard to response time, detection limit, selectivity, and dependences on temperature and pH as well as operating and storage stabilities. The sensor shows marked sensitivity to eight phenolic compounds usually present in industrial waste waters. The detection limit for phenol was obtained with 0.25 nM. Comparing sensor responses with DIN and EPA standard methods for the chemical analyses of both synthetic and real sample matrices, the results indicate the feasibility of the strip sensor for sensitive determination of phenols in field analysis.Phenolic compounds are important but toxic starting materials in a broad range of chemical manufacturing processes. Especially in coal conversion industry, phenolic residues are considered an acute enviromental problem. Often soil and surface waters of areas around former production and processing plants are contaminated by phenolic compounds with risk to ground water resources. Therefore, screening, monitoring, and control of these pollutants are of great importance.Photometric analyses by DIN and EPA standard methods12 are now commonly used for the determination of phenols. These procedures usually require sample pretreatment by filtration and distillation. Consequently, a simple and fast alternative method for the determination of phenols is desirable. For this purpose, biosensors based on phenol-oxidizing oxygenases appear promising; tyrosinase (EC 1.14.18.1) and as laccase (EC 1.10.3.2) are suitable receptor compounds, although the catalyzed reactions are different.* 123 Basic studies of physicochemical properties and catalyzed pathways of tyrosinase have shown that the tetrameric enzyme, + Umweltforschungszentrum Leipzig-Halle. * Institut ftir Chemo-und Biosensorik. § Bundesforschungsanstalt fur Landwirtschaft. (1) Deutsche Einheitsverfahren zur Wasser-, Abwasser-, und Schlammuntersuchung Band III; Summarische Wirkungs und Stoffkenngrdssen (Gruppe H); DIN 38409-H16; Bestimmung des Phenolindex; VCH: Weinheim, Germany; Jun 1984.(2) Methods for chemical analysis of water and wastes; EPA manual 600/4-79-020, Method 420.1:
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