Hydrogen Ion Role on the Reduction of Poly-(Neutral Red)
Authors: Agrisuelas Jeronimo , Cuenca Alejandro , Ferrus David , Frau Antonello , García Jose , Vicente Francisco 
Published: February 2020

Neutral Red (3-amino-7-dimethylamino-2-methylphenazine, abbreviated as NR) is a heterocyclic compound belonging to the phenazine family.
Like its close relative Azure A, NR has been subject of much ongoing research [1] due to its biochemical (as dye for staining in histology) and chemical (as a pH indicator) relevance. Among other relevant functions, NR can be used as DNA-revealing dye by optical and electrochemical methods [2], for the evaluation of natural and synthetic biomaterials [3, 4], development of optical sensors [5], mediator of electro transfer in organic and bio-electrochemistry [6]. The first polymer of such a family, poly(thionine) was obtained by Albery et al. [7] and, since then, a whole lot of research has been devoted to polymer derivatives of NR and phenazine-like compounds. Their potential applications span from photogalvanic cells [8], batteries [9], bio-sensing [10], electrochromic systems [11], sensor of pH [12] and biosensors metal/PNR [13].
Indeed, the presence of the counter-anion A- becomes important. When no small anions (i.e. Cl-) are present, the electro-catalysis is favoured at basic pH. On the other hand, H2 is discharged at acidic pH, because the big counter-anions present in the Britton-Robinson buffer cannot pass through the polymer matrix. In other words, the proposed mechanisms above can be summarized as follows:
PNR + 2e- + 2H+ ⇆ PNRH2

PNRH2 + 2 e- + 2 H+ ⇆ PNRH2 + H2

In order to retrieve reliable information about such redox processes and be able to tell “which is which”, derivative voltoabsorptometry was carried out on PNR films over the 2.18 to 8.95 pH range. Such an analytical method is a very useful tool in that UV-Vis absorptions are monitored as a function of a specific potential range. This, in turn, allows telling apart different contributions to the same redox peak(s) that may come from different phenomena. dA/dt changes against potential. During cycling of PNR at acidic pH, the most prominent changes were detected at 4 wavelengths: 418 nm, 587 nm, 745 nm, and 925 nm. In here, the current intensities and dA/dt are plotted against the potential window.
The reduction-oxidation processes of the central ring on the NR molecule [14] absorbs at 418 and 578 nm, whereas the redox activity of the inter-monomer bond causes absorption at 745 and 925 nm. It can be inferred that, at acidic pH, the central ring is protonated and its reduction is easier (i.e. a lower potential is required); as the pH increases, the amount of protonated PNR decreases and its reduction becomes more difficult (i.e. the potential shifts to more negative values) till it becomes impossible or highly unlikely/not detectable.
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ACKNOWLEDGEMENTS
Part of this work was supported by FEDER-CICyT CTQ2015-71794-R.

Figure 1