c. Some electrochemical and photoelectrochemical properties of 3-Amino-7-dimethylamino-2-methylphenazine (Neutral Red) in aqueous solution

Abstract

Neutral Red has been shown to undergo electrochemical and photoelectrochemical reactions in aqueous solution. At a pH above the pK,, the dye was oxidized with the loss of one proton with each electron. Similarly the protonated dye was oxidized at pH values below the pK,. Over the pH range examined, 1.0-9.2, Neutral Red was reduced with one proton being gained with each electron. The neutral radical which was formed could be further reduced. When the pH was above the pK, the dye was reduced by an H e e H mechanism, and when the pH was below the pK, the reduction proceeded by an H e H e mechanism with the possibility of further protonation.On a platinum electrode, Neutral Red underwent a photoelectrochemical oxidation at pH 7.2. The voltammogram showed a broad peak centred at 0 V (s.c.e.).Neutral Red, viz. 3-amino-7-dimethylamino-2-methylphenazine, has long been flyJbfe used as a pH indicator in biological studies'because its red-yellow colour change at h H, pK, 6.7 falls in the pH range close to that Neurrai Red of biological media. However, problems were encountered in studies of photosynthetic reactions when Neutral Red was used as a spectroscopic probe of the internal pH of thylakoidse3 The observed changes in the probe's absorbance were consistent with acidification of the medium but were much larger than expected. One explanation is that the dye was protonated preferentially in the aqueous phase; this indicated a variation of activity with l~c a t i o n .~ Other explanations for these results are that the unexpected decrease in the absorbance was due to Neutral Red acting as an artificial electron donor or acceptor in the photosynthetic reactions, and that the decrease was due to photoelectrochemical reactions of Neutral Red. As the dye absorbs light, it may undergo a charge-transfer reaction with species generated in the photosynthetic reactions.In concentrated acid solutions Havemann et aL4 examined solutions of sn2 + ions and Neutral Red at two pH values. At pH 0.5, where 75 "/,f the dye was in the

The mechanism of neutral red-mediated microbial electrosynthesis in Escherichia coli: menaquinone reduction

“…The second peak had a more positive midpoint potential of −346 mV Ag/AgCl , but with a much larger overpotential, demonstrated by the V pc of −909 mV Ag/AgCl . This second reduction peak was noted by Halliday et al (Halliday and Matthews, 1983), but has received little attention in the literature. Constant polarization on the cathodic side of both peaks (−650 mV Ag/AgCl and −950 mV Ag/AgCl ) yielded two solutions with different optical absorbance spectra (Fig.…”

“…Neutral red (3-Amino-7-dimethylamino-2-methylphenazine hydrochloride) is a cationic dye that is commonly used as a histological stain because of its optical sensitivity to pH in the physiologically relevant range of pH 6.0–8.0 (Halliday and Matthews, 1983). Although it is water-soluble, it has a lipid/water partition coefficient of 30 (LaManna and Lauro, 1998), making it extremely hydrophobic.…”

Electrogenerated chemiluminescence 73: acid–base properties, electrochemistry, and electrogenerated chemiluminescence of neutral red in acetonitrile

“…There have been several reports on the electrochemistry of neutral red in aqueous solution [22][23][24], mainly focused on the electrochemical reduction of NRH + , but the electrochemistry of NR and NRH + in an organic solvent has not been previously reported, even though neutral red was first synthesized in 1878 [25]. Fig.…”

Poly(neutral red): Electrosynthesis, Characterization, and Application as a Redox Mediator

“…It should be noted that pH 0.5 and 11.5 are only significant as the limits of experimental information. The electrochemical reactions leading to polymer formation are represented schematically in Figure 2, as suggested in [27]. The radicals obtained during NR oxidation initiate the polymerization process and the number of radicals formed defines the structure of the polymer: the more radicals formed, the more branched the polymer obtained [28].…”

“…The pK a values of the deprotonation reactions were determined as 6.7 for Reaction 6, 6.3 for Reaction 7, and 4.4 for Reaction 8 [26]. Combining this information on the protonation of NR and leuco-NR, the following overall reactions are obtained for the reduction of NR in different pH regions [27]:…”