The previously developed prototype ISFET (ion-sensitive field effect transistor)-based coulometric sensor-actuator system suffers from delay in response due to the nonzero distance between the sensor and actuator. The authors describe a novel configuration of a sensor-actuator device which employs a porous noble metal electrode as the actuator closely spaced around and over the gate of the ISFET. The novel device was characterized by performing an acid-to-base titration. The results showed that the delay time was considerably reduced. The titration times were found to be from 0.1 to 4.0 s. The corresponding concentrations of titrated species were from 0.5 to 15 mM.
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