Poly methylpyrrole Films

The electrochemical behavior of the conducting polymer poly(methylpyrrole) (PMPy) is shown in Fig. 3.21. Starting at approximately O.OV/SCE, the polymer is oxidized. Thereby the cationic sites are 

FIGURE 3.20. Volta-potential differences between three electrolytes and a coated electrode withdrawn from these electrolytes Electrode potential OV/SCE. The 

The results of Volta-potential measurements on the emersed PMPy-coated electrodes are summarized in Fig. 3.22, which presents the results at various electrode potentials derived from the preceding measurements. The increasingly positive slope indicates that the polymer assumes the state of an anion exchanger as it is oxidized. In Fig. 3.23 the expected Donnan potentials were calculated with Eqn. 18 the polaron/bipolaron density, obtained by integrating the oxidation current in Fig. 3.21, was assumed to constitute the fixed-charge density c. At the electrolyte concentration = 0.1 M, the calculated and experimental values (see Fig. 3.22) were matched, and at lower concentrations, theoretical and experimental values were plotted. The agreement can be considered quite good. Based on this analysis. Fig. 

FIGURE 3.21. Cyclic voltammograms of a poly(iV-methylpyrrole) film on glassy carbon in 0.1 M KCl aqueous electrolyte. Film thickness approximately 0.2 fim electrode area 0.5 cm potential sweep rates are indicated (v). 

24 is an appropriate representation of the membrane states of PMPy in reduced and oxidized states. 

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As for the case in PAn, the conductivity of PPy and poly(N-methylpyrrole) films is also strongly affected by the applied potential in liquid SO2 [171]. Van Dyke et al [172] reported that the potential domain in which PPy is conductive can be extended in non-aqueous media, e.g., acetonitrile. Normally, the PPy films are conductive above about —0.4 V vs. SCR but this can be extended down to about —1.1 V by simply treating the doped film with sodium hydroxide. This effect was attributed to the shift in the reduction potential of hydoxide-doped PPy because of its large association constant with oxidized pyrrole sites. 

On the other hand direct electron transfer between the poly(pyrrole) and GOx has been claimed. Subsequent work on GOx immobilized in poly(N-methylpyr-role) suggests that the process is very inefficient unless the enzyme is partially denatured during immobilization. In this study there was no evidence to support direct oxidation of the enzyme for films grown at 25° C. When films were grown at 50° C, the authors found evidence of direct oxidation of the enzyme, and they observed that selectivity of the enzyme was significantly reduced. They rationalize these observations by suggesting that at 50° C, the enzyme is partially denatured, so it is entrapped within the poly(N-methylpyrrole) film in a more open form, allowing access of polymeric chains to the active site of the enzyme. 

Detailed studies of glucose oxidase immobilized in poly(N-methypyrrole) films using both ferrocene monocarboxylic acid and ferricyanide as the mediator species indicate that in both cases poly(N-methylpyrrole) films are insulating and there is no evidence of direct reoxidation of the mediator on the polymer. This is an unexpected result, since these mediators are electroactive at poly(N-methylpyrrole) films that do not contain GOx. It appears that in addition to catalyzing the oxidation of glucose in the presence of a mediator, GOx can also catalyze the reduction of oxygen to H2O2 in the presence of mediators (M) as electron acceptors... 

In addition to poly(pyrrole) and poly(N-methylpyrrole), other conducting polymers have been used for enzyme immobilization, including poly(thio-phene), poly(indole), and poly(aniline). " Poly(thiophenes) and poly (indole) films have the possible disadvantage that the enzyme/polymer film must be grown from aprotic solvents, conditions that may denature the enzymes. The... 

Bartlett, P.N., Z. Ali, and V. Eastwick-Field. 1992. Electrochemical immobilization of enzymes. Part 4. Co-immobilization of glucose oxidase and ferro/ferricyanide in poly(N-methylpyrrole) films. 

York, F.T.A Schuermans, B.C.A.M. Barendrecht E. Influ ce of inserted anions on the properties of polypyrrole. ElectrochimActa 1990.35,567-75. Prezyna, L. A. Qiu, Y.-J. Reynolds, J. R. Wnek, G. E. Interaction of cationic polypeptides with electroactive polypyrrole/poly(styrmesulfonate) and polyCN-methylpyrrole)/ poly(styrenesulfonate) films. Macromolecules 1991,24,5283-5287. 

Ferraris and Itanlon electrochemically polymerized (68) in propylene carbonate using LiC104 as a supporting electrolyte [124], The film grew rapidly but also quickly dissolved off the ITO electrode when the applied potential was removed. It was possible to prepare films of sufficient insolubility for optical measurements. Poly(68) films could be reversibly cycled between 1.8 and 4.1 V versus Li. Electrical conductivity was 280 Scm. These results were compared with those of polymers from 2,5-bis(2-thienyl)-furan, V-methylpyrrole and thiophene. 

Moreover, selectivity could be also enhaneed if a specific interaction between the immobilized catalyst and the substrate in solution were to occur. A significant example is that of the voltam-metric detection of NO in the rat brain from a carbon fiber microelectrode modified with a [(H20)Fe PWii039] -containing poly(N-methylpyrrole) film and a Nation outer layer [150]. The good selectivity of this sensor was attributed not only to the Nafion membrane, which constituted an efficient electrostatic barrier against anionic interferents, but also to the formation of a metal-nitrosyl complex between the heteropolyanion and NO. The in vivo NO measurements were validated by injecting the rat with an NO-synthase inhibitor, which led gradually to the disappearance of the NO oxidation peak (Fig. 7). 

A nitrite-sensitive material has been developed by Fabre et al. with a poly(iV-methylpyrrole) film incorporating a metal-substituted heteropolyanion [(H20)Fe XWn039]" (X = P, n = 4, or X = Si, n = 5) as a doping anion [38C1-382]. Such a film was electrochemically stable and exhibited an efficient elec-trocatalytic activity vis-a-vis the nitrite reduction. In contrast, poor results were obtained when PPy was used as the immobilization matrix [383, 384]. The key step of this electrocatalytic process was the formation of an iron-nitrosyl complex generated from the replacement of H2O initially coordinated to the iron center by an NO group, the reduction of which led to the catalytic conversion of NO2 into ammonium ions [385, 386]. The measured catalytic currents were linear with the nitrite concentration over the range 1 X 10 to 3 X 10 M [382]. Furthermore, anions such as NOJ,... 

The poly(l-methylpyrrole) films and PEDOT films doped with 2-(V-morpholino) ethanesulfonic acid (PMPy-MES and PEDOT-MES films) were deposited potentiostatically from 0.4 or 1.33 M MES and 0.1 M MPy or 0.01 M EDOT solutions. The applied potential was equal to 750 mV for PMPy film deposition and 1050 mV for PEDOT film deposition, and the deposition time was (usually) equal to 600 s. 

Clearly, if the concentration of polymer sites is not a function of E, the rate of mediated reduction must also be constant. This behavior is easily demonstrated experimentally (245). If one reduces Fe(CN)6 cathodically on an electrode coated with a poly(N-methylpyrrole) film, one obtains the rotation-dependent, but largely potential-independent, reduction currents represented in Fig. 20.48. 

The use of polypyrrole and its derivatives was investigated soon after the reported discovery of polyaniline membranes. Liang and Martin [65] demonstrated that thin films of polypyrrole could be grown on alumina (An-opore) support membranes by using interfacial polymerization techniques. Doped polypyrrole films were found to be porous, showing Knudsen diffusion with an O2/N2 selection coefficient of 0.94. However, poly(A/-methyl-pyrrole) films were nonporous and showed good gas transport and selectivity. For example, a Ajxm poly(A/-methylpyrrole) film doped with NOs" ions had an oxy-... 

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

Other workers have prepared poly(V-methylpyrrole)/poly(biphenol-A-carbon-ate) (PC) using this approach.68 The electrodes were dip-coated with the PC and then electropolymerization was induced. Thermogravimetric analysis verified that a graft copolymer was produced. A similar procedure has been used to prepare PAn composites with the same host polymer.69 The in situ electrochemical polymerization process has also been used to prepare polyacrylonitrile/PPy composite films.70... 

F. Estrany, R. Ohver, E. Armelin, H.I. Iribaren, F. Liesa, and C. Aleman, Electroactive properties and electrochemical stability of poly(3,4-ethylenediox3dhiophene) and poly(n-methylpyrrole) multi-layered films generated by anodic oxidation. Port. Electrochim. Acta, 25, 55-65 (2007). 

The H2O2 produced by this reaction can react with the poly(N-methylpyrrole) and destroy the conductivity of the film. 

In devices of the first type, the conducting polymer starts in the oxidized state and is reduced as a result of the enzyme-catalyzed reaction of the substrate. This changes conductivity of the polymer, thus switching on or off the device. To reset the switch, it is necessary to connect the device to a potentiostat and reoxidize the film electrochemically (Fig. 9.14). Two devices have been reported to date, one based on poly(pyrrole)/poly(N-methylpyrrole) and responsive to NADH, the second based on poly(aniline) and responsive to glucose. In the latter case the device is switched from off to on, which appears to be advantageous, since it gives much faster response times. 

Metal-oxide-semiconductor FETs (MOSFETs) were also prepared by electrochemical polymerization using PPy and N-alkyl substituted PPy films, including poly (N-methylpyrrole) (PNMePy) and poly (N-ethylpyrrole) (PNEtPy) as a p-type semiconductor and p-toluenesulfonic acid monohydrate as a supporting electrolyte [168]. Figure 8.93 represents the cross-sectional view of the fabricated MOSFET. The mobility of PPy and PNEtPy FETs was 1.7 cm s, which is close to the value of silicon inorganic transistors [168]. 

Not only do oxidatively doped polypyrrole films show significant environmental stability, but a number of these films can also be electrochemically prepared and switched in both organic and aqueous solvents [90]. While poly(N-methylpyrrole) was the only polymer of the series that exhibited a comparable electrochromic contrast in both solvents, N-benzyl (19a), N-tolyl (19b), and the parent polypyrrole showed a decrease in electrochromic contrast when comparing results obtained using organic and aqueous electrolytes. Polymers prepared using N-phenyl and N-benzoylpyrrole (20) showed no electrochromic switching in aqueous solutions. 

Massoumi and Entezami [92] reported the controlled release of dexamethasone sodium phosphate (DMP) from a conducting polymer bilayer film consisting of a PPy inner film doped with DMP and poly(N-methylpyrrole)/polystyrene sulfonate (PNMP/PSS) or polyaniline sulfonate (SPANI) outer film. DMP was released from the inner film by an application of less than —0.6 V. In this device, the outer polymer layer functions as an ion and solvent barrier and also effectively reduces the rate of DMP release under an applied reducing electrochemical field, thereby providing an additional route to controlling release rates. 

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