Polypyrrole current

FIGURE 6-20 Configuration of a penicillin sensor based on an microarray electrode coated with a pH-responsive polypyrrole. Vq = gate voltage VD = drain voltage ID = drain current PS = potentiostat CE and RE = counter and reference electrodes, respectively. (Reproduced with permission from reference 76.)... 

The flow of an anodic current oxidizes the conducting polymer and the film swells. At the polypyrrole/tape interface, electrochemically stimulated conformational changes in the polymer promote an expansion that... 

Figure 25. Movement rate of bilayer devices (along an angle of 90°) with different dimensions (different polypyrrole weights) versus applied electrical current per mass unit (mA mg ). (Reprinted fromT. F. Otero and J. M. Sansinana, Bilayerdimensions and movement of artificial muscles. Bioelectrochem. Bioener-genetics 47, 117, 1997, Fig. 4. Copyright 1997. Reprinted with permission from Elsevier Science.)...

At dusk the window becomes lighter. When the polypyrrole film is completely reduced and the oxide is fully oxidized and darkening continues, the current of the photocell decreases at it and the electric light in the room is switched on. The intensity of the electric current sent to the lamp is increased in such a way that the luminosity in the room remains constant at all times. In cars or for other applications, the device can work automatically or by hand, darkening all the windows when the car is parked on a sunny day. 

Polymerization at constant current is most convenient for controlling the thickness of the deposited film. Charges of ca. 0.3, 0.2, and 0.08 C cm-2 are required to produce 1 fim of polypyrrole,59 poly(3-methylthio-phene)60 (no data are available for polythiophene), and polyaniline 43 respectively. Although these values can reasonably be used to estimate the thicknesses of most electrochemically formed conducting polymer films, it should be noted that they have considerable (ca. 30%) uncertainties. For each polymer, the relationship between charge and film thickness can... 

Figure 17. Plots of (a) current end (b) change in mass vs. potential for a polypyrrole film in propylene carbonate containing 0.1 M UCIO ...

If the film is nonconductive, the ion must diffuse to the electrode surface before it can be oxidized or reduced, or electrons must diffuse (hop) through the film by self-exchange, as in regular ionomer-modified electrodes.9 Cyclic voltammograms have the characteristic shape for diffusion control, and peak currents are proportional to the square root of the scan speed, as seen for species in solution. This is illustrated in Fig. 21 (A) for [Fe(CN)6]3 /4 in polypyrrole with a pyridinium substituent at the 1-position.243 This N-substituted polypyrrole does not become conductive until potentials significantly above the formal potential of the [Fe(CN)6]3"/4 couple. In contrast, a similar polymer with a pyridinium substituent at the 3-position is conductive at this potential. The polymer can therefore mediate electron transport to and from the immobilized ions, and their voltammetry becomes characteristic of thin-layer electrochemistry [Fig. 21(B)], with sharp symmetrical peaks that increase linearly with increasing scan speed. 

Besides synthesis, current basic research on conducting polymers is concentrated on structural analysis. Structural parameters — e.g. regularity and homogeneity of chain structures, but also chain length — play an important role in our understanding of the properties of such materials. Research on electropolymerized polymers has concentrated on polypyrrole and polythiophene in particular and, more recently, on polyaniline as well, while of the chemically produced materials polyacetylene stih attracts greatest interest. Spectroscopic methods have proved particularly suitable for characterizing structural properties These comprise surface techniques such as XPS, AES or ATR, on the one hand, and the usual methods of structural analysis, such as NMR, ESR and X-ray diffraction techniques, on the other hand. 

Materials such as polypyrrole are exciting in terms of their future technological impact, not just because of the obvious applications of such a simple, cheap electrochromic but because it may be possible to develop them sufficiently to replace the more expensive, and often toxic, metallic conductors commonly employed in the electronics industry. This may not be such a distant dream since it has been calculated that the intrinsic conductivity of these materials, i.e. without the defects that are currently defeating attempts to increase their conductivity of c, < lOOOfl 1 cm", may be many times that of copper. 

Hence the authors concluded that bipolarons are the major current carriers on the basis of the calculation showing them to be 0.49 eV more stable than two polarons, with polarons combining where possible to give bipolarons. This paper dominated the interpretation of experiments on polypyrrole for several years until in situ techniques became routinely applied,... 

The broad nature of the current peaks in the voltammogram of conducting polymers such as poly pyrrole has been interpreted in a number of w one of which was to attribute it to the movement of anions across the polymei, electrolyte interface, a vital process if the overall charge neutrality of the film is to be maintained. The participation of the electrolyte in the electrochemistry of the polymer film is easily seen by comparing the response of polypyrrole in a variety of different electrolytes (see Figure 3.74). 

Figure 3.74 Cyclic voltammograms of a 20nm-thick polypyrrole film on Pt in CHjCN containing different electrolytes. Two sweep rates are shown, the voltammograms showing the tower currents were taken at 50mVs , and the larger currents were obtained at lOOmVs"...

Electron transfer of the glucose oxidase/polypyrrole on the electrode surface was confirmed by differential pulse voltammetiy and cyclic voltammetry. The glucose oxidase clearly exhibited both reductive and oxidative current peaks in the absence of dissolved oxygen in these voltammograms. These results indicate that electron transfer takes place from the electrode to the oxidized form of glucose oxidase and the reduced form is oxidized by electron transfer to the electrode through polypyrrole. It may be concluded that polypyrrole works as a molecular wire between the adsorbed glucose oxidase and the platinum electrode. 

Electrochemical communication between electrode-bound enzyme and an electrode was confirmed by such electrochemical characterizations as differential pulse voltammetxy. As shown in Fig. 11, reversible electron transfer of molecularly interfaced FDH was confirmed by differential pulse voltammetry. The electrochemical characteristics of the polypyrrole interfaced FDH electrode were compared with those of the FDH electrode. The important difference between the electrochemical activities of these two electrodes is as follows by the employment of a conductive PP interface, the redox potential of FDH shifted slightly as compared to the redox potential of PQQ, which prosthetic group of FDH and the electrode shuttling between the prosthetic group of FDH and the electrode through the PP interface. In addition, the anodic and cathodic peak shapes and peak currents of PP/FDH/Pt electrode were identical, which suggests reversibility of the electron transport process. 

Cyclic voltammetry was performed with the ADH-NAD-MB/polypyrrole electrode in 0.1 M phosphate buffer (pH 8.5) at a scan rate of 5 mV s l. The corresponding substrate of ADH caused the anodic current at +0.35 V vs. Ag/AgCl to increase. These results suggest a possible electron transfer from membrane-bound ADH to the electrode through membrane-bound NAD and MB with the help of the conductive polymer of polypyrrole. 

FIG. 23. Charge discharge curves for nanotubular (a) and thin film (b) LiMn204/ polypyrrole electrodes. Current density = 0.1 mA cm. Electrolyte was 1 M LiC104 in 1 1 (vol.) propylene carbonate dimethoxyethane. 

Although the diffusion of the counterion is faster in polypyrrole than in polyacetylene, its value is still low enough to influence the rate of the electrochemical charge and discharge processes of lithium/polymer batteries. Indeed the current output of these batteries is generally confined to a few mA cm . Possibly, improvements in the electrode kinetics, and thus in the battery rates, may be obtained by the replacement of standard ... 

Polypyrrol is a polymeric support that can be used in immobilization of ONDs to surfaces. The generation of polypyrrol films can be by electrochemical co-polymerization of pyrrole and pyrrole-modified ONDs onto platinum electrodes. The polymer forms a black and insoluble film that is electrically conducting and whose thickness depends on the current used during the polymerization process (Fig. 14). The final surface density of the OND can be controlled by the ratio of pyrrole/OND being polymerized [53-55]. 

In potentiometric sensors, an electrical potential between the working electrode and a reference electrode is measured at zero current conditions in a solution containing ions that exchange with the surface. The first potentiometric MIP sensor was prepared in 1992 by Vinokurov (1992). The substrate-selective polyaniline electrode was electrosynthesized with polypyrrole, polyaniline, and aniline-p-aminophenol copolymers. The development of an MIP-based potentiometric sensor was reported in 1995 by Hutchins and Bachas (1995). This potentiometric sensor has high selectivity for nitrite with a low detection limit of (2 + l)x 10 M (Fig. 15.10). 

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