Meldola Blue, oxidation

Oxidation of oxalic acid with dimethyl-V,V-dichlorohydantoin and dichloroisocya-nuric acid is of first order with respect to the oxidant. The order with respect to the reductant is fractional. The reactions are catalysed by Mn(II). Suitable mechanisms are proposed.129 A mechanism involving synchronous oxidative decarboxylation has been suggested for the oxidation of a-amino acids with l,3-dichloro-5,5-dimethylhydantoin.130 Kinetic parameters have been determined and a mechanism has been proposed for the oxidation of thiadiazole and oxadiazole with trichloroiso-cyanuric acid.131 Oxidation of two phenoxazine dyes, Nile Blue and Meldola Blue, with acidic chlorite and hypochlorous acid is of first order with respect to each of the reductant and chlorite anion. The rate constants and activation parameters for the oxidation have been determined.132... 

Modified electrodes for this analytical purpose have mostly been formed by electrode adsorption of the mediator systems on the electrode surface or by electropolymerization [24,116]. Recently, for example, NAD(P)H oxidations have been performed on platinum or gold electrodes modified with a monolayer of pyrroloquinoline quinone (PQQ) [117] or on poly(methylene blue)-modified electrodes with different dehydrogenases entrapped in a Nafion film for the amperometric detection of glucose, lactate, malate, or ethanol [118]. In another approach, carbon paste electrodes doped with methylene green or meldola blue together with diaphorase were used for the NADH oxidation [119]. A poly(3-methylthio-phene) conducting polymer electrode was efficient for the oxidation of NADH [120]. By electropolymerization of poly(aniline) in the presence of poly(vinylsulfonate) counterions. 

Coupling between a biologically catalyzed reaction and an electrochemical reaction, referred to as bioelectrocatalysis, is the constructional principle for enzyme-based electrochemical biosensors. This means that the flow of electrons from a donor through the enzyme to an acceptor must reach the electrode in order for the corresponding current to be detected. In case a direct electron transfer between the active site of an enzjane and an electrode is not possible, a small molecular redox active species, e.g. hydrophobic ferrocene, meldola blue and menadione as well as hydrophilic ferricyanide, can be used as an electron transfer mediator. This means that the electrons from the active site of the enzyme reduce the mediator molecule, which, in turn, can diffuse to the electrode, where it donates the electrons upon oxidation. When these mediator molecules are employed for coupling of an enzymatic redox reaction to an electrode at a constant potential, the resulting application can be referred to as mediated amperometry or mediated bioelectrocatalysis. 

Various redox compounds that fulfil catalyst characteristics have been investigated in systems with recycling of NAD by electrocatalytic methods. Quinones, formed either by oxidation of carbon surfaces [143, 145] or adsorbed to the electrode surface [146, 147], phenazines [148, 149], phenoxazine derivatives such as Meldola Blue [182], 9-naphthoyl-Nile Blue [151, 152] and l,2-benzophenoxazine-7-one [153], and also the organic conducting salt N-methyl phenazinium tetracyanoquinodimethanide (TTF TCNQ") [154, 155], ferricinium ions [156, 157] and hexacyanoferrat(IIl) ions [158, 159] can act as catalysts for the electrochemical oxidation of NADH. It is assumed that in corresponding electron-transfer reactions a charge-transfer complex between the immobilized mediator and NADH is formed. The intermediate reduced redox mediator will be reoxidized electrochemically. Most systems mentioned, however, suffer from poor electrode stabilities. 

Gorton, L., Torstensson, A., Jaegfeldt, H., Johansson, G., Electrocatalytic Oxidation of Reduced Nicotinamide Coenzymes by Graphite Electrodes Modified with an Adsorbed Phenoxazinium Salt, Meldola Blue , / Electroanal. Chem. 161 (1984) 103-120. 

The highest reaction rates were obtained for the mediators with the most oxidative E -values. A linear correlation was found when plotting the logarithm of the rate coefficients versus the E° -vaJues. The commercially available phenoxazine Meldola Blue, with an E -value at pH 7.0 of —185 mV vs SCE was found to be the most efficient one having a rate constant of 3 x at this pH. ... 

Further studies [67] of enzyme/polypyrrole systems have focused on modification of the enzyme. It was found that the redox dye, Meldola blue, forms a strong complex with alcohol dehydrogenase. It is also known that this dye makes the electrochemical regeneration of the coenzyme NADH possible [68,69]. By electropolymerizing pyrrole, Meldola blue, alcohol dehydrogenase and NAD a membrane was prepared that oxidized ethanol apparently by a direct transfer of electrons to the electrode. 

Fig. 8 Structural formulas of some commonly used mediators for catalytic NADH oxidation (a) Meldola blue (p-phenylenediimine) [220, 252] ...

Khayyami et al. used Meldola blue (MB) as electrrMi mediator.Although MB allows working at —0.1 V vs. Ag/AgCl, they apphed a working potential of -1-0.25 V in order to accelerate the oxidation rate. At this potential, the background interference was higher, but the utility of the mediator was proved, since unwanted contributions to the signal from other electroactive compounds were ntinimized. 

An alternative biosensor system has been developed by Hart et al. [44] which involves the use of the NAD+-dependent GDH enzyme. The first step of the reaction scheme involves the enzymatic reduction of NAD+ to NADH, which is bought about by the action of GDH on glucose. The analytical signal arises from the electrocatalytic oxidation of NADH back to NAD+ in the presence of the electrocatalyst Meldola s Blue (MB), at a potential of only 0Y. Biosensors utilising this mediator have been reviewed elsewhere [1,17]. Razumiene et al. [45] employed a similar system using both GDH and alcohol dehydrogenase with the cofactor pyrroloquinoline quinone (PQQ), the oxidation of which was mediated by a ferrocene derivative. 

Mention should be made of oxazine dyes, used also as biological stains, which are oxidized phenoxazine derivatives containing suitable auxochromic groups. A detailed treatment of these dyes, however, is beyond the scope of this review. Most of the industrial phenoxazine dyes are derived from benzophenoxazines (e.g., Meldola s blue) or from more complex ring systems containing the phenoxazine residue (triphendioxazine dyes).3,118 The long-known dyestuffs orcein and litmus which are prepared by the action of ammonia on certain lichens, and may also occur accidentally in nature, are both based on the oxidized phenoxazine ring system as shown by Musso and co-workers.119... 

Figure 39. The mechanism of ADH/NAD/MB-based biosensor response for ethanol ( Reprinteed from J. Electroanalytical Chemistry, 547, A.S. Santos, R.S. Freire, L.T. Kubota, Highly stable amperometric biosensor for ethanol based on Meldola s blue adsorbed on silica gel modified with niobium oxide, 137, Copyright(2003) with permission from Elsevier.

Mediator-chemically modified electrodes have been coupled either with ADH membranes to give enzyme electrodes (Cenas et al. 1984) or with ADH reactors, e.g., in an FIA device (Huck et al. 1984). Quinoidic groups, Meldola s Blue, and Nile Blue have been used as mediators. Albery et al. (1987b) employed an electrode containing NMP+ and TCNQ" in a PVC carrier for NADH oxidation. ADH was entrapped on the sensor surface by a dialysis membrane. 

In a recycling system using glutamate dehydrogenase and alanine aminotransferase the direct oxidation of the liberated NADH at an electrode modified with Meldola s Blue as well as the NADH oxidation by NMP+ and the subsequent detection of the oxygen consumption by... 

In addition, SPEs surfaces have also been covered with a wide variety of substances bismuth oxide, Prussian Blue, ferrocyanide, Meldola s Blue, Co-phthalocyanine, or some enzymes, in order to obtain suitable transducers for specific anal5d es. 

L-lactate Lactate oxidase and meldola s blue adsorbed on Si02 particles coated with niobium oxide and dispersed in a cartxm paste electrode Bio- and electro- catalysis [205]... 

Hydroxy-3//-phenoxazin-3-one Me ether, 10-oxide, in H-00443 Meldola s blue Chloride, in M-OOOl 1 5-Methoxy-11-methyl-9/J-benzo[a] phenoxazin-9-one, M-00089 11 -Methyl-9//-benzo[fl]phenoxazin-9-one, M-00140... 

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