In this study, we surface-functionalized the extended floating gate (FG) sensing surface (Al₂O₃) of an ion-sensitive floating-gate field-effect transistor (ISFGFET) with polyaniline/dinonylnaphthalenesulfonic acid (PANI/DNNSA) to construct an organo-surface functionalized bio-sensing device. We measured the threshold voltage (V_T) of the device, which was used to determine the potential change at the interface between the PANI/DNNSA layer and aqueous solution. The device was characterized under various pHs as well as enzymatic reaction catalyzed by horseradish peroxidase (HRP) immobilized on the PANI/DNNSA functionalized floating-gate sensing surface. Interestingly, upon surface modifications, the pH and enzymatic reaction sensing behaviors of the device became dependent on the PANI/DNNSA layer, proving that open circuit potentiometric (OCP) responses can be transferred to ISFGFETs. Kinetics of the reactions was tuned through the selection of substrates, exemplified with o-Phenylenediamine (OPD) and 2,2′-Azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS). The potential shifts caused by those reactions were explained using an operational model that illustrates “dual sensitivity” of PANI/DNNSA toward ionic and redox species. The sensing concept for the probing of peroxidase-catalyzed charge/electron transfers can potentially be applied to hydrogen peroxide (H₂O₂) sensing and paves the way for multiplexed potentiometric ELISA tests on such an ion-sensitive field-effect transistor (ISFET) device.