A conventional biosensor is composed of closely coupled biological sensing and signal processing elements. The biological sensing element converts a change in an immediate environment to a signal conducive to processing by the signal processing component. The characteristics of an ideal biosensor are presented in Figure 1.1 and, as shown, include speci city-especially in complex matrices; fast measurements-in millisecond ranges that are consistent with biological signaling timescales, such as the duration of a nerve cell action potential; small size-to provide portability as well as in vivo implantation capabilities; continuous measurement or reversibility-to provide multiple use with little sensor regeneration or renewal; electronic processing-or integration with other devices or into larger systems; and sensitivity-in picomolar ranges that are consistent with biological detection ranges, such as concentrations of signaling molecules (e.g., cyclic AMP).