Fine-Tuning an ISFET-Based Dual pH–Total Alkalinity Sensor for Operation in Seawater Using an Adjustable, Suspended Anode

In this study, we present a re-envisioned design of an ion-sensitive field-effect transistor (ISFET)-based solid-state sensor for measuring pH and total alkalinity of seawater with electrolytically generated titrant. The original design requires either custom nanofabrication of ISFET wafers or back-end processing of fully fabricated ISFETs. Instead, we assembled all "off-the-shelf" parts to demonstrate the same measurement principle but with the titrant-generating electrode suspended orthogonal to the pH-sensing region of the ISFET rather than being physically deposited on the face of the chip. This offers many benefits including (1) avoiding disruption the pH functionality of the ISFET; (2) enabling the assessment of anode-gate distance sensitivity from a single ISFET; and (3) exploration of different electrode geometries and composition without complex ISFET fabrication. In this study, we analyzed the sensitivity of the A_T measurement to the spacing of the titrant-generating electrode (anode) to the pH-sensing region of the ISFET (gate) as well as sensitivity to the electric current applied to the anode at three seawater A_T compositions. The greatest sensor resolution in seawater was achieved when operating at a low anode current (5 μA) and greater anode-gate distance (∼150 to 200 μm) for the suspended anode configuration. The suspended anode configuration also showed an improvement in equivalence point determination with sharper inflections relative to the on-chip anode configuration. By averaging (<i>N</i> = 5), a sensor error of 2.85 μmol kg^-1 was achieved over 131 measurements.