Nile Blue, oxidation

The oxidation of N ADH has been mediated with chemically modified electrodes whose surface contains synthetic electron transfer mediators. The reduced form of the mediator is detected as it is recycled electrochemically. Systems based on quinones 173-175) dopamine chloranil 3-P-napthoyl-Nile Blue phenazine metho-sulphatemeldola blue and similar phenoxazineshave been described. Conducting salt electrodes consisting of the radical salt of 7,7,8,8-trtra-cyanoquinodimethane and the N-methylphenazium ion have been reported to show catalytic effects The main drawback to this approach is the limited stability... 

At the physiological level it is well established that vital dyes such as nile blue, neutral red and methylene blue retard larval development under normal lighting conditions (12L/12D with source unspecified) (25 27). Female but not male pupal weights are also reduced. Unfortunately experiments were conducted without dark controls so that it is difficult to evaluate the role of photosensitization in these effects. As house flies and fire ants succumb to photosensitization, they lose motor control and become more excitable (28). This suggested a neurotoxic effect and investigation of fire ant acetylcholinesterase vitro revealed that this enzyme was sensitive to photo-oxidation. vivo results, however, revealed no effect on the enzyme which suggests another mode of action. Epoxldatlon of cholesterol and membrane lysis may be alternative primary sites. If this were the case ecdysone metabolism of insects would probably also be effected. 

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... 

A number of methods for determining Mo have been based on its catalytic effect in oxidation of iodide by hydrogen peroxide [112-118], or in reduction of some organic substances, e.g. Nile Blue [119], Toluidine Blue [120], Metanilic Yellow [121], Safranine [122], and Nile Red [123] by suitable reducing agents. 

Schelter-Graf et al. (1984) coupled an LDH column reactor with electrochemical NADH indication. These authors increased the electrochemical selectivity by using a graphite electrode modified with 3- 3-naphthoyl-Nile Blue which oxidizes NADH at a potential of -0.22 V vs SCE. In an FLA system a sample frequency of 15/h with a relative standard deviation of 1% was obtained. 

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. 

Figure 14-9. Cyclic voltammogram of a graphite electrode modified with P-naphthoyl Nile Blue, a) buffer (pH 8.0) b) after addition of NADH (10 mM). The increase in the anodic current is attributed to electrocatalytic oxidation of NADH at the mediator-modified electrode. - SOO to +300 mV vs. SCE 5 mV s" 0.1 M phosphate buffer (pH 8.0) with 0.5 M NaCl graphite disk electrode, 6.4 mm diameter.

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. 

Most adsorbed phenoxazine derivatives give slightly broader peaks than what is theoretically stipulated for a Langmuirian adsorption process. The number n, whether evaluated from the o.5 or from eqn.(l), is usually somewhat lower than two, reflecting interactions between the adsorbed molecules. The immobilization process may strongly influence the properties of the adsorbed species, which is clearly revealed for Nile Blue in Fig.3. Both the energy level, as well as the various pKa-values of the oxidized and the reduced forms, are changed on adsorption. " ... 

Two examples may be given, by reacting the reduced form of Nile Blue with 1,5-dichloro-2,5-dinitrobenzene, one compound could be obtained with an E -value of —135 mV vs SCE (pH 7.0), and by reacting the iminoform of Brilliant Cresyl Blue with terephthaloyl chloride a compound could be obtained with an E -value of —55 mV vs SCE (pH 7.0). They were both found to be catalytically active for NADH-oxidation. No kinetic data are available yet, however. Purification and identification of both the commercially available phenoxazines (usually with a purity of 50-90 %) and of the reaction products are far from straightforward. 

Cai CX, Xue KH (1997) Electrocatalysis of NADH oxidation with electropolymerized films of nile blue A. Anal Chim Acta 343(l-2) 69-77. doi 10.1016/ s0003-2670(96)00592-2... 

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