PPy films

Figure 19. (1) Oxidized polypyrrole (PPy) film electrogenented on a steel electrode. (2) A tape was fastened to the dry polypyrrole film (A). B is doublesided tape and C is a protective sheet of paper. (3) The bilayer device with a protective film is removed from the electrode. (4) The protective sheet is peeled off and the bilayer is ready to work. (Reprinted from Handbook of Organic Conductive Molecules ami Polymers, H.S. Nalwa, ed.,Vol. 4,1997, Figs. 10.13, 10.15a, 10.18, 10.36. Reproduced with permission of John Wiley Sons, Ltd., Chichester. UK.)...

In 1979, Diaz et al. produced the first flexible, stable polypyrrole (PPy) film with high conductivity (1(X) Scm ). The substance was polymerized on a Pt-electrode by anodic oxidation in acetonitrile. The then known chemical methods of synthesis " usually produced low conductivity powders from the monomers. By contrast, electropolymerization in organic solvents formed smooth and manageable films of good conductivity. Thus, this technique soon gained general currency, stimulating further electropolymerization experiments with other monomers. In 1982, Tourillon... 

Independently of this, chronoabsorptiometric measurements by Genies et al. have proved that PPy films grow in timer linear to t and not to j/t. In the opinion of the authors this implies that the rate-determining step during film growth is a radical ion coupling and not the diffusion of the uncharged monomer towards the electrode surface. The attested phenomenon that PPy polymerizes... 

Fig. 11. Cyclic voltammetry of first discharging/charging of galvanostatically prepared PPy films (PC, 0.5 M LiC104) the first three cycles between +0.5 V and —0.3 V, the following between +0.5 V and —1.1 V...

Garcia et al., 1998 [107] Fructose Dietetic products Fructose 5-dehydrogenase/in a polypyrrole (PPY) film Platinum electrode/ +0.25 mV vs. Ag/AgCl Sodium ferricyanide... 

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. 

Spectroscopic Measurements. A Beckman Model 5230 spectrophotometer was used to record in situ UV-visible spectra of the PPy films, which were electrochemically deposited on the indium-tin oxide (ITO) coated glass (Delta Technologies). For Raman measurements a Spex Model 1403 double spectrometer, a DM IB Datamate, and a Houston Instrument DMP-40 digital plotter were employed. Details of the experimental setup for in situ Raman spectroscopy are described elsewhere (26). 

Cyclic Voltammetric Behavior of the PPy-GOD Film. Figure 1 shows the cyclic voltammetric curves of a PPy-GOD film (4000 A) in phosphate buffer solution with pH 7.4 at different scan rates. Both anodic and cathodic peaks should correspond to the redox reactions of PPy chains. The peak potentials, which were recorded at the scan rate of 200 mV/s, were -380 mV and -200 mV for cathodic and anodic peaks, respectively. This is similar to the potential shifts of the PPy film doped with large anions (27) such as poly(p-styrenesulfonate). Enzyme protein molecules are composed of amino acid and have large molecular size, which can not move out freely from the PPy-GOD film by the application of the reduction potential. In order to balance the charge of the Pfy-GOD film, cations must move into the film, and redox potentials move toward a more negative potential. This behavior is different from the one observed for the PPy-GOD film, which was prepared in the solution of GOD... 

UV-Visible and Raman Spectroscopies. In situ UV-visible absorption spectra of a 5000 A PPy-GOD film, which was formed on an ITO coated glass, were recorded in the PB solution (pH 7.4). The spectra recorded at both the oxidation (0.4 V) and the reduction (-1.0 V) potentials showed an absorption peak near 380 nm, which is due to the PPy. When the PPy was reduced at -1.0 V, the absorbance in the wavelength range of 500-800 nm decreased, and the absorbance at 380 nm increased. The observed spectral changes of the PPy-GOD film during the redox reaction were similar to those of the PPy film doped with C104" (PPy-C104) (27). 

Effect of Overoxidation. It has been reported that the PPy film doped with small anions can be overoxidized by applying 1.0 V or higher and the... 

Cycling of the PPy-film through its reduced and oxidized state does not lead to any loss of the flavin containing polyanion. In the case of low molar mass flavin-containing dopants most of the flavin is released from the film upon reduction of the PPy. 

Influence of polymerization conditions upon incorporation of flavin-containing polyanion. The amount of polyanion (1) incorporated as dopant in a PPy film during electropolymerization can easily be controlled by changing the ionic strength (low molar mass salt) and/or the pH of the monomer solution. The correlation between the amount of polymer-bound flavin incorporated in the film and the concentration of the added low molar mass salt, sodium-p-toluenesulphonate (NaOTs), at pH=7 is shown... 

Figure 2. Current transients for the growth of PPy films at several (constant) potentials at pH=8.0...

The PPy-films doped with (1) were characterized by cyclic voltammetry (Figure 6) and UV/VIS-spectroscopy (Figure 7). The immobilized polymer-bound flavin moieties showed very good electrochemical activity with El4= -0.496 V (vs SCE), which is a little more negative than values (-0.45 V) found in the literature for free flavins (34). If we compare the UV/VIS-spectra (on ITO-electrodes, Figure 7) of... 

Figure 7. UV/VIS-spectra of PPy films doped with polyanion (1) or NaOTs (inset aqueous solution of (1) at pH=8)...

PPy-films doped with either NaOTs or polymer (1), they appear to be quite similar. Both show the typical absorbance profile of polypyrrole. The latter also shows two characteristic bands of the flavin units bound to polymer (1) (XtallUK==438 and 335 nm). 

The thickness of the PPy-film has an effect on the response of the electrode towards the substrate, and can be controlled by the amount of charge passed through the system during synthesis of the conducting polymer. As can be seen in Table I, the response of the electrode to BNAH (slope of the calibration curve) increases with the thickness of the film up to about three Coulombs of charge passed. If thicker layers are deposited the response is only slightly lowered. This suggests that the transport of electrons from the (reduced) flavin to the electrode does not depend upon the diffusion of a reactive species (H to the platinum surface, which would limit the current as the film thickness is increased. 

The incorporated flavin-containing polyanions are immobilized very efficiently in the polypyrrole film. Figure 10 shows the current response curve of PPy-films doped with polymer (1) or with a low molar mass species, 3-(p-methylbenzoic acid)-7,8,10-trimethylisoalloxazine (2) respectively, for a freshly prepared film and for a PPy-film... 

Coating by a thin layer of PPy has been realized on multiwalled nanotubes (MWNT) [29,91,93], well-aligned MWNT [85] and single-wall nanotubes (SWNT) [88], When MWNT are oxidized, their surface is covered with oxygenated functionalities, which can be used as anionic dopant of a PPy film electrodeposited on the MWNT [94], These films are notably less brittle and more adhesive to the electrode than those formed using an aqueous electrolyte as source of counterion. 

The effectiveness of PQQ entrapment and the dependence of the electrode response on the PPy film thickness were investigated by the application of cyclic voltammetry. The PPy/PQQ modified electrode was immersed in 10 or 20 mM solution of the thiol and potential was scanned from -0.5V to 0.5V monitoring the resulting current. 

It has been shown that the thickness of the polypyrrole (PPy) film has a significant effect on the electrode performance.17 Figure 6 shows the dependence of the response of PPy/PQQ modified electrode to 10 mM DMAET (A) and 10 mM DEAET (B) as a function of PPy film thickness. As film thickness increases the oxidation current increases for both DMAET and DEAET, presumably due to increases in the amount of PQQ loaded in the PPy film. The maximum current for the oxidation of PQQH2 is observed when 200 nm films are used. When the PPy film thickness was larger then 200 nm a decrease in the sensor response was observed, which could be due to increased resistance (R ) of the thicker film. The optimum 200 nm PPy film thickness was used to characterize the performance of the electrode for amperometric detection of thiols. 

Figure 6. The dependence of the oxidation current generated by PQQH2 as a function of PPy film thickness in presence of (A) 10 mM DMAET and (B) 10 mM DEAET.

PPy film is blue-violet in doped (oxidized) stet. Electrochemical reduction yields the yellow-green undoped form. The schematic of the doping/dedoping process can be given as... 

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