Device electrochemical switching

The ability to switch a molecular unit on and off is a key component of an efficient molecular device, since it allows modulation of the physical response of such a device by external physical or chemical triggers. A molecular device, based on a trinuclear metal complex, shown in Figure 59, functions as an electroswitchable-photoinduced-electron-transfer (ESPET) device.616 Electrochemical switching of the redox state of a spacer intervening between a donor-acceptor pair can dictate the type of the observable charge separation and the lifetime of the resulting ion pair.616... 

S. L. Gilat, S. H. Kawai, and J.-M. Lehn, Light-triggered molecular devices photochemical switching of optical and electrochemical properties in molecular wire-type diarylethene species, Chem. Eur. J1, 275-284 (1995). 

Figure 3.53 Electrochemical cell employing flexural-wave device as one electrode. Top Cell with RF source to drive platinum-covered flexural-wave device in bottom of cell. Middle Limiting celt current vs time as both transducers on FPW device are switched on and off, producing standing waves, at the different drive amplitudes shown as parameter. Bottom Square-law dependence of increase of limiting current upon transducer voltage that produces mixing of the liquid in the cell. (Reprinted with permission, see Ref. (76). 1991 IEEE.)...

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. 

With biomedical applications in mind, this chapter reviews the important elements of the synthesis and processing of conducting polymers as well as their fabrication into devices. The key properties that make the use of ICPs in biomedical applications an attractive proposition are their electronic and electrochemical switching properties. These important features will be discussed with specific emphasis upon their use as sensors or as actuators from the biomolecular to the biomechanical levels. 

Due to its many advantageous properties (low cost, fast color change, good contrast, stability, etc.), PANI is also a favorite material for use in electrochromic display devices. Pictures of a PANI-based flexible device are shown in Fig. 7.11. The display pattern, which consists of 25 pixels and the connections that allow each pixel to be driven separately, was fabricated by depositing gold onto a plastic sheet. Another plastic sheet covers the display. The electrochemical switching is executed using a counterelectrode, which also serves as a reference electrode, and an acidic gel electrolyte is placed between the two sealed plastic sheets. 

Leroux, Y. R. Lacroix, J. C. Chane-Ching, K. 1. Fave, C. Felidj, N. Levi, G. Aubard, J. Krenn, J. R. Hohenau, A. Conducting polymer electrochemical switching as an easy means for designing active plasmonic devices. J. Am. Chem. Soc. 2005, 127, 16022-16023. 

Some applications of polypyrrole, such as drug delivery devices, require specific ion transport. Control of the ion transport process requires determination of the identity of the mobile ion, which is complicate by the fact that there are several ions present in the system. Moreover, Shimidzu and co-woiicers [138,139] demonstrated that ion transport in polypyrrole can be modified by the use of polymeric anions as dopants. Miller and Zhou [140] proved that the electrochemical switching of polypyrrole could achieve controlled release of small anions (such as CIO4), while the incorporation of an immobile polyanion (such as polystyrene sulfonate) resulted in cation transport. The polyanions become trapped within the polypyrrole matrix due to their large size and, perhaps more important, their entanglement with the polypyrrole chains. This increases the stability and mechanical strength of polypyrrole and improves electrical conductivity and electroactivity [141,142]. Therefore it has been of significant interest in the polymerization of pyrrole in polyanion electrolyte solutions [143,144]. 

Changes in volume have been described during electrochemical switching of polypyrrole in some specific conditions [297,300], which have been applied to microactuators [419]. Moreover, these microscopic movements can be transformed into macroscopic ones by the construction of a bilayer structure. These systems can act as electrochemopositioning devices, their movements being controlled by the applied electrical potential [420 23]. 

The future prospects for polymer electrolytes look promising because it has been appreciated that they form an ideal medium for a wide range of electrochemical processes. Other than primary and secondary batteries, and high and low temperature fuel cells, practical applications for polymer electrolytes that are under consideration include electrochromic devices, modified electrode/sensors, solid-state reference electrode systems, supercapacitors, thermoelectric generators, high-vacuum electrochemistry and electrochemical switching. Device applications that have been the main driving force behind the development of polymer electrolytes are treated hereafter. 

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