Microelectrochemical devices

Recently, several molecule-based microelectrochemical devices have been developed by the Wrighton group.(14.15.21-22) A microelectrode array coated with poly(I) results in a microelectrochemical transistor with the unique characteristic that shows "turn on" in two gate potential, Vq, regimes, one associated with the polythiophene switching from an insulator to a conductor upon oxidation and one associated with the v2+ + conventional redox centers. 

The poly(I)-based transistor is the first illustration of a microelectrochemical transistor based on a combination of a conducting and a conventional redox polymer as the active material. The transistor "turns on" at VG corresponding to oxidation of the polythiophene backbone. The resistivity of poly(I) declines by a factor of 105 upon changing VG from 0.4 V to 0.8 V vs. Ag+/Ag. When Vg is moved close to the one-electron reduction potential of V2+/+, the conventional redox conductivity gives a small degree of "turn on". A sharp Iq-Vq characteristic results, with an Ip(peak) at Vq = E° (V2+/+). Though the microelectrochemical devices based on conventional redox conduction have both slow switching speed and a... 

Lambrecht M, Sansen W. Biosensors Microelectrochemical Devices. Institute of Physics Publishing, Bristol, Philadelphia, and New York, 1992. 

Fig. 13. Microelectrochemical device, (a) Schematic illustration of the microelectrochemical transistor based on polyaniline (thickness of the polyaniline layer 5 pm, electrode width 1-2 pm, distance 2-4 pm) (b) characteristic curve of the polyaniline transistor (Id versus Vg at Vd = 0.18 V). (Redrawn from Wrighton, 1986).

The application of modified electrodes can be exploited in such technologies as energy storage, microelectrochemical devices, supramolecular chemistry, elec-trochromic displays, electrocatalysis, solar energy conversion and electroana-lysis. ... 

SOLID-STATE MICROELECTROCHEMICAL DEVICES TRANSISTOR AND DIODE DEVICES EMPLOYING A SOLID POLYMER ELECTROLYTE... 

Not long ago,this group first described microelectrochemical devices, which are based on microfabricated arrays of electrodes, connected by electroactive materials. Because the active components of these devices are chemical in nature, many of these devices are chemically sensitive,and comprise a potentially useful class of chemical sensors. Devices showing sensitivity to pH, 02r 2 f and Na" have been demonstrated. These devices are, typically, operated in fluid solution electrolytes. If this class of devices is to be useful as gas sensors, systems which are not dependent on liquid electrolytes need to be developed. We have recently reported solid state microelectrochemical transistors, which replace conventional liquid electrolytes with polymer electrolytes based on polyethyleneoxide (PEG) and polyvinylalcohol (PVA). In this report, we discuss additional progress toward solid-state devices by employing a new polymer ion conductor based on the polyphosphazene comb-polymer, MEEP (shown below). By taking advantage of polymer ion conductors we have developed microelectrochemical devices, where all of the components of the device are confined to a chip. 

Figure 2. Top. Schematic of a p3MeT-based solid-state microelectrochemical device. Center. Cyclic voltammetry at the p3MeT derivatized electrodes. At left, the device is characterized in the solution electrolyte CH3CN/O.I M L1CF3S03 before the application of MEEP. At right, the same device is characterized under MEEP/LiCF3S03 (5 1). Bottom. Comparison of the steady-state vs. Vq of the p3MeT device in fluid solution electrolyte and under MEEP/LiCF3S03. Electrodes 3 and 4 are source and drain respectively (see Figure 1).

WO3 is an example of another class of electroactive material, metal oxides, which has been used to construct microelectrochemical devices. WO3 is a wide-band-gap semiconductor, with high resistance in its neutral state.Upon reduction, WO3 intercalates cations such as H" ", Li" ", and Na and becomes conducting. W03 based transistors, showing sensitivity to pH and to Li" concentration have been demonstrated in solution electrolytes. A schematic of a MEEP/WO3 device is shown in Figure 3. WO3 is confined to the required electrodes, using standard photolithographic techniques. 

Figure 3. Top. Schematic of a W03 based solid-state microelectrochemical device. Bottom. Steady-state vs. Vq of the device diagrammed at top. Electrodes 6 and 1 are source and drain.

Figure 5. Schematic and transistor characteristic of a new class of microelectrochemical device,which is based on a redox-active material dissolved in a polymer ion conductor. Here, TMPD is sublimed into and saturates the MEEP/LiCF3S03 film. The drain voltage, V j, is 25 mV.

In the near future, further development in new soft microelectrochemical devices based on conjugated polymer actuators will require their complete conception, operation, and interfacing systems all integrated on a single substrate to allow easy and independent operation. This still poses a real challenge that remains to be overcome. Also, it would be of great interest to develop microactuators with linear actuation to further extend the tool box of CP microactuator devices. 

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