Realization of a PEDOT:PSS/Graphene Oxide On-Chip Pseudo-Reference Electrode for Integrated ISFETs

Abstract

A stable reference electrode (RE) plays a crucial role in the performance of an ion-sensitive field-effect transistor (ISFET) for bio/chemical sensing applications. There is a strong demand for the miniaturization of the RE for integrated sensor systems such as lab-on-a-chip (LoC) or point-of-care (PoC) applications. Out of several approaches presented so far to integrate an on-chip electrode, there exist critical limitations such as the effect of analyte composition on the electrode potential and drifts during the measurements. In this paper, we present a micro-scale solid-state pseudo-reference electrode (pRE) based on poly(3,4-ethylene dioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS) coated with graphene oxide (GO) to deploy with an ion-sensitive field-effect transistor (ISFET)-based sensor platform. The PEDOT:PSS was electropolymerized from its monomer on a micro size gold (Au) electrode and, subsequently, a thin GO layer was deposited on top. The stability of the electrical potential and the cross-sensitivity to the ionic strength of the electrolyte were investigated. The presented pRE exhibits a highly stable open circuit potential (OCP) for up to 10 h with a minimal drift of ~0.65 mV/h and low cross-sensitivity to the ionic strength of the electrolyte. pH measurements were performed using silicon nanowire field-effect transistors (SiNW-FETs), using the developed pRE to ensure good gating performance of electrolyte-gated FETs. The impact of ionic strength was investigated by measuring the transfer characteristic of a SiNW-FET in two electrolytes with different ionic strengths (1 mM and 100 mM) but the same pH. The performance of the PEDOT:PSS/GO electrode is similar to a commercial electrochemical Ag/AgCl reference electrode.

Keywords: 2D materials; PEDOT:PSS; biosensor; diffusion barrier; gate electrode; stability.

Figure 1

Photograph of the used electrode test structure (a). An image of a PEDOT:PSS electrode surface (b). SEM image of a PEDOT:PSS electrode coated with GO (c). High-resolution image of a GO-coated PEDOT:PSS electrode showing micro holes in the GO film (d) Microscopy image showing the SiNW arrays, an integrated temperature sensor, and an on-chip pRE (e). SEM image of a single SiNW-FET (f).

Figure 2

Bode plots of electrochemical impedance spectra (left) and phase (right) of an Au electrode, a PEDOT:PSS-coated Au electrode, and a GO-coated PEDOT:PSS electrode.

Figure 3

OCP measurements of four different electrodes against an electrochemical Ag/AgCl RE. pRE 4 exhibits the lowest drift, while the other electrodes exhibit an unstable OCP, especially within the first hour.

Figure 4

Impact of the addition of higher ionic strength droplets on the OCP of pRE 2 and pRE 4. The black arrows indicate the addition of high ionic strength solution.

Figure 5

Transfer characteristics at different pH of a SiNW-FET gated with different gate electrodes (a). Threshold voltage change of a SiNW-FET due to changes in pH (b). Schematic illustration of the remaining cross-sensitivity and the reason for the relatively high standard deviation of our pRE (c).

Figure 6

SEM image of the optimized GO coating (a). Multiple transfer characteristic measurements at different pH (b). Comparison in threshold voltage change due to changing the pH from 7 to 10 for an optimized pRE4 and a commercial Ag/AgCl RE (c).