Pyrrole films

Figure 3.81 Typical cyclic voltammograms of a poly pyrrole film on Pt in Nrsatu rated 1 M NaClOj. The voltammograms were collected immediately after holding the film at -0.6 V vs, SCE for 5 min and after cycling for 5 min. The scan rate was 100 mV s "1 and the film thickness 84 nm. Reprinted from Electrochimica Acta, 36, P.A, Christensen and A. Hamnett, In situ Spectroscopic Investigations of the Growth, Electrochemical Cycling and Overoxidation of Polypyrrole in Aqueous Solution , pp. 1263-1286(1991), with kind permission from Pergamon Press Ltd., Headington Hill Hall, Oxford OX3 0BW, UK.

A poly(pyrrole) film was deposited on a Pt electrode from potentiostatic conditions at 0.8 V vs Ag/AgCl. The film was colorless, its presence was verified by oxidation and reduction of the film in plain electrolyte solution. The infrared spectrum of the electrochemically prepared poly(pyrrole) is similar to the catalytically prepared films indicating the two films are structurally similar. 

The use of nitroxyls that are amenable to immobilisation in a polymer layer around the anode would be an ideal way of constraining the catalyst where it can be regenerated electrochemically. Attempts to utilise a poly(pyrrole) film formed by anodic oxidation of the monomer 5 led to a system that will oxidise alkanols but which is unstable in continuous use [40], A more satisfactory polymer layer is... 

As described in Section 3 of Chapter 4, the stabilization of n-Si electrode by coating with poly(pyrrole) has attracted much attention. The stabilization of a small bandgap n-semiconductor electrode against oxidation is of great value not only to convert visible light into chemical energy, but also to construct liquid-junction solar cells operated under visible irradiation. The poly(pyrrole) film is usually electropolymerized on the semiconductor electrode dipped in the aqueous solution of pyrrole. The remarkable stabilizing effect of poly(pyrrole) film on polycrystalline n-Si is shown in Fig. 22 67). The photocurrent obtained under irradiation in the aqueous solution of... 

The electrochemical behavior of poly(pyrrole) films prepared and cycled in an AICI3 [C2mim][Cl] melt was investigated in detail and improvements in reproducibility and the rate of oxidation and reduction of these films were observed compared to films prepared under similar conditions in acetonitrile [49]. This was postulated to be a result of an increase in the porosity of poly(pyrrole) films deposited from the melt compared to those from acetonitrile, although attempts to describe this porosity using porous electrode models were not totally conclusive. 

This appears to be contradictory to the smoother poly(pyrrole) films that are formed in air- and water-stable ionic liquids [46, 51]. 

Fig. 7.5 Poly(pyrrole) films grown in [C4mpyr][NTf2] (a), [C2mim][NTf2] (b) and PC/Bu4NPF6 (c), constant potential onto Pt.

Poly(pyrrole) films grown by constant potential in the ionic liquid tri(hexyl) (tetradecyl) phosphoniumbis(trifluoromethanesulfonyl)amide, [P6,6,6,u][NTf2], and subsequently cycled 100 times and reduced in the monomer-free ionic liquid, showed the presence of both ionic liquid cations and anions within the film. Interestingly, poly(pyrrole) films grown by constant potential in the ionic liquid but with no subsequent electrochemical cycling also contained both ionic liquid cation and anion within the film (Figure 7.19). 

However, the DC conductivity of the pyrrole film was sufficiently high to indicate that the polymer chain was intact and this precludes the possibility of any chemical reaction between the cation and the polymer. The successful expulsion of cations from the film (see below) also eliminates this possibility. The 19F NMR spectrum (Figure 7.19(b)) shows the presence of intercalated anions within the film, as indicated by the peak at —82 ppm that is consistent with [NTf2] . The 13C NMR spectrum ofthe film (Figure 7.19(c)) shows a broad resonance from the poly(pyrrole) centered at ca. 120 ppm, and also significant intensity between 0 and 30 ppm from the alkyl chains of phosphonium cations within the film [106]. 

Fig. 18.8. Doping and undoping of a poly(pyrrole) film during oxidation and reduction cycles. When a poly(pyrrole) coated electrode is cycled between the reduction and oxidation potentials, the current observed at the oxidation potential is related to the ability of anions to enter the polymer film and dope the polypyrrole.

Recognizing that the conducting polypyrrole films can be chemically modified (.1,2), the phenyl substituent assumes a particularly important role because it provides a means of introducing a wide selection of functional groups into the polymer. With this objective in mind, we have prepared a series of N-arylpyrrole polymers and find the thin poly-N-(p-nitrophenyl)pyrrole films of particular interest because they combine the electroactive properties of nitrobenzene and polypyrrole. With this combination, the polymer can be switched electrochemically between the cationic, neutral, and anionic form. 

FIGURE 2.7 Fine-structured poly(pyrrole) film formed on the ionic liquid surface using 10 ms voltage pulses. (Source Dr. Jenny Pringle, Monash University, Australia.)... 

K. Bouzek, KM. MaDgoId, and K. Jiittner, Electrocatalytic activity of platinum modified poly-pyrrole films for the methanol oxidation reaction, J. AppL Electrochem., 31, 501-507 (2001). 

P.R. Solanki, S. Singh, N. Prabhiar, M.K. Pandey, and B.D. Malhotra, Apphcation of conducting poly(aniline-co-pyrrole) film to cholesterol biosensor, J. Appl. Polym. ScL, 105(6), 3211-3219 (2007). 

Preparation of Gas Sensors Based on Poly(pyrrole) Films... 

Many redox enzymes can be denatured if the pH of the solution is taken too far from neutral or if nonaqueous solvents are used. It is therefore desirable, although not always essential, to use neutral aqueous solutions in the electropolymerization process, thus restricting the choice of monomers that can be used. One way to overcome this problem is to incorporate, or immobilize, the enzyme after deposition of the polymer film. This allows a much wider choice of deposition conditions. In addition to adsorbing the enzyme onto preformed poly(pyrrole) films, " GOx has also been covalently immobilized onto a wide range of preformed 3- and N-derivatized pyrrole carboxylates or onto N-amino-substituted pyrroles. This technique may increase current densities at a given glucose concentration by as much as twentyfold when compared to the best results obtained elsewhere. The authors make no claims for direct electron transfer between enzyme... 

Evidence against a direct electrochemical mechanism for enzyme reoxidation was given in a detailed study of GOx immobilized in poly(pyrrole) films by Belanger et who showed that the conductivity of the film is destroyed during... 

Electropolymerization of phenols proceeds similarly to that for pyrrole poly(phenol) itself is probably a mixture of para- and meta-linked units. Films are generally continuous and free from such defects as pinholes. They have been used for corrosion protection, as permselective films, " and as pH sensors. Like poly(pyrrole) films, electropolymerized phenol films have a number of features that make them attractive for immobilizing an enzyme. First these films can be grown under electrochemical control from aqueous buffered solution at neutral pH. Second a wide variety of phenol derivatives are available that allow some control over the films physical characteristics. Third these films are permselective, which could be useful in preventing interfering species from reaching the... 

FIGURE 9.15. Schematic representation of an enzyme switch responsive to penicillin based on penicillinase immobilized on a poly(pyrrole) film (see Ref. 97). On adding penicillin protons are produced, which cause a local decrease in the solution pH and hence an increase in the conductivity of the polymer. 

In the study of NADH oxidation at poly(pyrrole), Schuhmann and colleagues used poly(pyrrole) and modified poly(pyrrole) films. For poly(pyrrole) itself and poly(pyrrole) containing electrostatically bound ferricyanide, vanadate, or molybdate anions, they observed no NADH oxidation, although these electroactive anions are known to oxidize the NADH in homogeneous solution. By using pyrrole modified by covalent attachment of chloranil or 2,3-dichloro-1,4-naphthoquinone (Fig. 9.19) structures, they were able to obtain catalytic oxidation of NADH and reasonable stability. It this case quinone functionalities are presumed to act as catalytic sites for oxidation, since they are known to be efficient homogeneous oxidants for NADH. ... 

The electrochemistry of the PQQ (pyrrolo-quinoline quinone) prosthetic group has been investigated at poly(pyrrole)-coated electrodes, and PQQ has been entrapped within poly(pyrrole) films " good electrochemistry was observed in both cases. Poly(pyrrole) has also been used to entrap adenosine triphosphate (ATP) anions, again by growing the film in the presence of the anion, " and as an electrode material for oxidizing ascorbate. " In the latter case the oxidation... 

---