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.