Modification of Microelectrode Arrays: New Microelectrochemical Devices for Sensor Applications [and Discussion]

Microelectrode arrays consisting of two or eight closely spaced (ca. 1 $\mu$ m) gold or platinum microelectrodes (approximate dimensions 2 $\mu$ m wide x 50 $\mu$ m long x 0.1 $\mu$ m high) can be modified with a variety of redox-active materials in a way that leads to microelectrochemical devices that may be useful as sensors. In general, a pair of microelectrodes connected by a redox-active material represents a device, because the current, I$_d$, passing from one microelectrode (source) to the other (drain) at a fixed potential between them, V$_d$, can be modulated by electrically or chemically induced changes in the state of charge of the redox material. The fundamental issues associated with pH-sensitive devices based on poly (3-methylthiophene), poly(4-vinylpyridine), a viologen-quinone redox polymer and WO$_3$ are described to illustrate generic concepts that underlie the development of microelectrochemical sensors.