Enhancing Potentiometric Response Using Modified Ion Sensitive Transistor
Authors: Abhinav Vishnuram , Patkar Rajul , Vinchurkar Madhuri , Naik Tejas Rajendra , Baghini Maryam Shojaei 
Published: February 2020

High sensitivity, high selectivity, reproducibility and quick response are of prime importance for ion detection in various applications. In this respect, electrochemical detection is the best technique as compared to the traditional flame photometry, ion chromatography and surface plasmon resonance [1]. In an ion sensitive field effect transistor (ISFET), directly exposing the gate oxide of field effect transistor (FET) to an ionic solution causes ion penetration into the oxide layer. This is reflected in I-V characteristics that show voltage-dependent hysteresis due to ion penetration [2]. Extended gate field effect transistor (EGFET) shows great potential over the usual methods of ISFETs or ion sensitive electrode (ISE). EGFET effectively improves the reliability by separating the sensor and the transducer, associating the two by interconnect. It also provides a wide range of detection with possibility for device miniaturization and is extremely easy to fabricate [3].
In this work, we present a novel combination of solid-state ISE and FET integrated on same platform. Thus, creating an EGFET. We have built an electrochemical sensing unit comprised of all solid-state electrodes, an Ag/AgCl reference electrode and ionophore coated gold electrode. Unlike any of the earlier reports, all the electrodes are integrated with a FET in a single plane of printed circuit board (PCB), hence mitigating the issues of separate wire bonding or external connection between the sensing element and the transducer by using PCB traces [2]. In this work, we demonstrate for the first time, the use of ISE in EGFET configuration without degrading the linearity and sensitivity due to the external voltage bias. Potentiometric measurement on our potassium ion (K+) ISE shows near Nernstian limit sensitivity (49 mV/decade), low concentration for the limit of detection (10−6 M), and linearity over a large range of detection (10−4 M to 1 M).
We demonstrate a simple approach to develop an array of miniaturized two terminal planar solid state electrode device consisting of an ion selective membrane (ISM) on gold as the working electrode (WE) and solid state silver/silver chloride (Ag/AgCl) as the reference electrode (RE), both clamped to the metal contacts of the FET. 18-crown-6 (ionophore) has been utilized in ion selective membrane on WE to achieve high selectivity towards K+ ions [4]. Highly stable and low cost Ag/AgCl electrode with the drift potential of less than 2mV is used as pseudo RE [5]. The principle mechanism behind our sensing approach is transduction of ionic stimulus into electrical signal. Concentration dependent change in the ISM potential at WE/electrolyte interface prompts a corresponding adjustment in overall gate potential, thus modifying the charge transport in FET. Sensitivity and linearity of potassium ion (K+) ISE is determined by potentiometric measurements at several concentrations ranging from 1M to 10−6 M. The electrodes are reusable after each measurement by simply rinsing it in deionized water. Our study shows that voltage biasing does not affect the electromotive force (Vcell). This makes the device suitable and robust for EGFET configuration to enhance the sensitivity for the ion detection applications. Two-fold increase in the sensitivity (100mV/decade, much higher than reported values ion selective electrodes [3,6]) is observed in EGFET configuration. The innovative approach used in our solid-state K+ EGFET has great potential to propel the future in micro-devices with commercialized product for the applications of clinical investigation, environmental and agricultural field. Our methodology represents a handy procedure by using facile fabrication technique for the accomplishment of multipurpose, versatile, and cost-effective ion sensing devices.
Reference

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Figure 1