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 AT 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 AT 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 (N = 5), a sensor error of 2.85 μmol kg–1 was achieved over 131 measurements.