Electrochemical polymerization polypyrroles

To increase the stability of the phenothiazine dye CMEs, investigations of the electrochemical reactions of redox proteins at polymer CMEs have been undertaken. One example of such a study is the heterogeneous redox reaction of Cyt c at an electrochemically polymerized polypyrrole-methylene blue (PPy-MB) film CME. Figure 17 shows the cyclic voltammetric response that occurs during the preparation of this CME by potential... 

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Nitration of the surface of polypyrrole and the subsequent reduction of the nitrate groups has been reported [244] and Bidan et al. [306, 307] have investigated the electrochemistry of a number of polymers based on pyrroles with /V-substituents which are themselves electrochemically active. Polypyrrole has also been successfully deposited onto polymeric films of ruthenium complexes [387], and has been used as an electrode for the deposition and stripping of mercury [388], As with most conducting polymers, several papers have also appeared on the use of polypyrrole in battery systems (e.g. [327, 389] and Ref. therein). 

The polypyrrole molecular interface has been electrochemically synthesized between the self-assembled protein molecules and the electrode surface for facilitating the enzyme with electron transfer to the electrode. Figure 9 illustrates the schematic procedure of the electrochemical preparation of the polypyrrole molecular interface. The electrode-bound protein monolayer is transferred in an electrolyte solution containing pyrrole. The electrode potential is controlled at a potential with a potentiostat to initiate the oxidative polymerization of pyrrole. The electrochemical polymerization should be interrupted before the protein monolayer is fully covered by the polypyrrole layer. A postulated electron transfer through the polypyrrole molecular interface is schematically presented in Fig. 10. 

A platinum disk electrode was electrolytically platinized in a platinum chloride solution to increase the surface area and enhance the adsorption power. The platinized platinum electrode was then immersed in a solution containing 10 mg ml l ADH. 0.75 mM and 6.2 mM NAD. After sufficient adsorption of these molecules on the electrode surface, the electrode was transferred into a solution containing 0.1 M pyrrole and 1 M KC1. Electrochemical polymerization of pyrrole was conducted at +0.7 V vs. Ag/AgCl. The electrolysis was stopped at a total charge of 1 C cm 2. An enzyme-entrapped polypyrrole membrane was deposited on the electrode surface. 

Apart from the insulating polymeric matrices, conductive polymers such as polypyrrole and polyaniline have been used as nanocomposite electrodes by chemical or electrochemical polymerization [13, 17, 116, 117]. Such materials provide high conductivity and stability. However, the use of insulating polymers can be more advantageous than the conductive polymers when employed in cyclic voltammetry. 

Similar approach has also been taken by Ferain and Legras [133,137,138] and De Pra et al. [139] to produce nanostructured materials based on the template of the membrane with etched pores. Polycarbonate film was also of use as the base membrane of the template, and micro- and nanopores were formed by precise control of the etching procedure. Their most resent report showed the successful formation of ultrasmall pores and electrodeposited materials of which sizes were as much as 20 nm [139]. Another attractive point of these studies is the deposited materials in the etched pores. Electrochemical polymerization of conjugated polymer materials was demonstrated in these studies, and the nanowires based on polypyrrole or polyaniline were formed with a fairly cylindrical shape reflecting the side wall structure of the etched pores. Figure 10 indicates the shape of the polypyrrole microwires with their dimension changes by the limitation of the thickness of the template. 

Thiophene, pyrrole and their derivatives, in contrast to benzene, are easily oxidized electrochemically in common solvents and this has been a favourite route for their polymerization, because it allows in situ formation of thin films on electrode surfaces. Structure control in electrochemical polymerization is limited and the method is not well suited for preparing substantial amounts of polymer, so that there has been interest in chemical routes as an alternative. Most of the methods described above for synthesis of poly(p-phenylene) have been applied to synthesise polypyrrole and polythiophene, with varying success. 

The electrochemical polymerization of pyrrole or thiophene readily lends itself to formation of composites. Polypyrrole-acetylene laminates have been made by using polyacetylene as an electrode 295). The polypyrrole forms as a 5 pm skin on the polyacetylene. If the polyacetylene is first doped, the polypyrrole completely permeates the film. In both cases the conductivity of the composite reached 30-40 S cm-1 and was much less sensitive than that of pure polyacetylene to exposure to moist air or water, so that the polypyrrole protects the polyacetylene even in the case where it permeates the film. In this latter case, treatment with ammonia caused the conductivity to drop by 30 x whereas for the sandwich films the conductivity dropped by 4600 x through the film but only 17 x in the surface layers. 

By electropolymerization of pyrrole in solvents containing polyelectrolytes such as potassium polyvinylsulfate, it is possible to prepare films of polypyrrole with polymeric counterions which have good conductivity (1-10 S cm-1) and strength (49 MPa) 303 304,305). Such a material could be used reversibly to absorb cations in an ion exchange system. Pyrrole has also been electrochemically polymerized in microporous polytetrafluoroethylene membranes (Gore-tex), impregnated with a perfluorosulphonate ionomer 3061. 

Comparable to thiophene, pyrrole is a five-membered heterocycle, yet the ring nitrogen results in a molecule with distinctly different behavior and a far greater tendency to polymerize oxidatively. The first report of the synthesis of polypyrrole (PPy) 62 that alluded to its electrically conductive nature was published in 1968 [263]. This early material was obtained via electrochemical polymerization and was carried out in 0.1 N sulfuric acid to produce a black film. Since then, a number of improvements, which have resulted from in-depth solvent and electrolyte studies, have made the electrochemical synthesis of PPy the most widely employed method [264-266]. The properties of electrosynthesized PPy are quite sensitive to the electrochemical environment in which it is obtained. The use of various electrolytes yield materials with pronounced differences in conductivity, film morphology, and overall performance [267-270]. Furthermore, the water solubility of pyrrole allows aqueous electrochemistry [271], which is of prime importance for biological applications [272]. 

Polypyrrole thin film doped with glucose oxidase (PPy-GOD) has been prepared on a glassy carbon electrode by the electrochemical polymerization of the pyrrole monomer in the solution of glucose oxidase enzyme in the absence of other supporting electrolytes. The cyclic voltammetry of the PPy-GOD film electrode shows electrochemical activity which is mainly due to the redox reaction of the PPy in the film. Both in situ Raman and in situ UV-visible spectroscopic results also show the formation of the PPy film, which can be oxidized and reduced by the application of the redox potential. A good catalytic response to the glucose and an electrochemical selectivity to some hydrophilic pharmaceutical drugs are seen at the PPy-GOD film electrode. 

In this paper we report the electrochemical polymerization of the PPy-GOD film on the glassy carbon (GC) electrode in enzyme solution without other supporting electrolytes and the electrochemical behavior of the synthesized PPy-GOD film electrode. Because the GOD enzyme molecules were doped into the polymer, the film electrode showed a different cyclic voltammetric behavior from that of a polypyrrole film doped with small anions. The film electrode has a good catalytic behavior to glucose, which is dependent on the film thickness and pH. The interesting result observed is that the thin PPy-GOD film electrode shows selectivity to some hydrophilic pharmaceutical drugs which may result in a new analytical application of the enzyme electrode. 

Figure 9 Electrochemical polymerization of polypyrrole LB film. (From Ref. 84.)...

The discovery that doped forms of polypyrroles conduct electrical current has spurred a great deal of synthetic activity related to polypyrroles [216-218], Reviews are available on various aspects of the synthesis and properties of polypyrroles [219,220]. In addition, summaries of important aspects of polypyrroles are included in several reviews on electrically conducting polymers [221-226]. Polypyrrole has been synthesized by chemical polymerization in solution [227-231], chemical vapor deposition (CVD) [232,233], and electrochemical polymerization [234-240]. The polymer structure consists primarily of units derived from the coupling of the pyrrole monomer at the 2,5-positions [Eq. (84)]. However, up to a third of the pyrrole rings in electrochemically prepared polypyrrole are not coupled in this manner [241]. 

Recent development in multilayer sensor architecture using sequential electrochemical polymerization of pyrrole and pyrrole derivatives to entrap enzymes was tested on a tyrosinase-based phenol sensor [127]. A phenothia-zine dye, thionine served as redox mediator and was covalently attached to the thin, functionalized first polypyrrole layer on Platinum disk electrodes. Then, a second layer of polypyrrole with entrapped tyrosinase was electrochemically deposited. The phenol sensor constructed in this manner effectively transferred electron from enz3Tne to the electrode surface. As all steps in preparation, including deposition of the enzyme-containing layer are carried out electrochemically, this technique may prove to be applicable for mass production of miniature sensors. 

Fig. 1.7 SEM image ofthe surface of an electrochemically polymerized film of polypyrrole p-toluene sulfonate.

Interesting supports are the polymeric materials, notwithstanding their thermal instability at high temperatures. In the electrocatalysis field, the use of polypyrrole, polythiophene and polyaniline as heteropolyanion supports was reported [2]. The catalytically active species were introduced, in this case, via electrochemical polymerization. Hasik et al. [3] studied the behavior of polyaniline supported tungstophosphoric acid in the isopropanol decomposition reaction. The authors established that a HPA molecular dispersion can be attained via a protonation reaction. The different behavior of the supported catalysts with respect to bulk acid, namely, predominantly redox activity versus acid-base activity, was attributed to that effect. 

Polymers in this category are of interest in that they can be easily synthesized, being of very high stability. A couple of these polymers are illustrated in Fig. 16. Poly(p-phenylene) is prepared by the polymerization of benzene with A1C13 and CuCl2 (Kovacic and Kyriakis, 1963 Kovacic and Oziomek, 1964), whereas polypyrrole and polythienylene are prepared by the electrochemical polymerization of each monomer with an appropriate electrolyte (Diaz et al., 1979 Tourillon and Gamier, 1982). Polythienylene is also synthesized by the polycondensation of dihalo-thiophene (Yamamoto et al., 1980). 

NMR spectroscopy will provide a simpler spectral pattern, when compared with NMR spectroscopy, because a given N resonance line may correspond to a given structure. Therefore, the structure of doped and dedoped N-labeled polypyrrole films can be successfully studied by high resolution solid-state NMR [14, 15]. Doped and dedoped samples were prepared by electrochemical polymerization [16] using 20-30% N-labeled pyrrole. To obtain a dedoped sample, the electrodes were inverted after the doping experiment and the same voltage applied. 

It is well Renown that organic conducting polymers such as polypyrrole, polythiophene, and polyaniline can be deposited on electrodes by means of electrochemical polymerization, which is successfully carried out through oxidation of monomers in the solution (14). 

Another way to functionalize depositions, is to entrap functional molecules in polypyrrole. Incorporation of functional molecules during electrochemical polymerization and the application of so prepared films to sensors have been studied by many workers (27). By means of area-selective deposition much more complicated functional depositions can be prepared on an electrode. The authors have shown that organic dyes (an example of functional molecules) are incorporated in the process of... 

The him morphology of electrochemically prepared polythiophene has been shown in numerous studies to be almost identical to that commonly observed for polypyrrole (described in Chapter 2). A nodular surface is observed for both unsubstituted and 3-alkyl substituted thiophenes.92 As with PPy, the electrochemical preparation of PTh at higher current densities produced rougher surface morphologies. The similarity in morphologies suggest a similar growth mechanism for electrochemically polymerized PPy and PTh. 

A polypyrrole membrane doped with anions of high molecular weight during electrochemical polymerization changes in anion permselectivity or cation perm-... 

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