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We report the realization of high-sensitivity ion-selective field-effect transistors (ISFETs) using nanoporous polysilicon on the gate region. Owing to the presence of a nanoporous film, the effective area of the exposed surface becomes larger than that of the channel area of a regular transistor. The response of such transistors to pH has been measured for a wide range from four to nine, showing a different behavior from regular ISFETs where a change in the threshold voltage is recorded. A relative current-based sensitivity can be adapted for such devices. A high sensitivity on the order of 300 mV/pH is reported, owing to the presence of 3-D nanostructures.