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.

Physical Interpretation of Mixed Ionic‐electronic Conductive Polymer‐coated Electrodes by a Simple Universal Impedance Model

“…Compared to bare noble metal electrodes, the deposition of polymers or the polymer mixture resulted in a broader resistive plateau and an overall lower impedance in the capacitive regime at low frequencies. The lowering in interfacial impedance due to polymer coatings is a well‐known effect [7a]. The direct comparison of the differently polymer‐coated electrodes furthermore reveals significant differences.…”

“…Electrodes with a diameter of 500 μm were fabricated as described earlier [7a]. Briefly, a metal stack based on a 30 nm thick titanium (Ti) adhesion layer, 220 nm gold (Au), and 50 nm Ti as protective layer were sputter deposited on 4’’ glass wafers.…”

“…The deposition process was terminated after a predetermined total charge was transferred. To further ensure highly reproducible coating results, a 3D‐printed fluidic container with insertion slots for the working electrode (WE) and counter electrode (CE) were used [7a]. The PEDOT : PSS or PPy : PSS layer were obtained by electropolymerizing EDOT or Py and PSS (both 50 % v/v).…”

“…The optimization of this process must be accompanied by suitable characterization techniques. One well‐established method to investigate the electrode properties is the so‐called electrochemical impedance spectroscopy (EIS) [7a, 10]. Typically, a sinusoidal signal with an amplitude in the mV regime and a varying frequency is applied to the device under test to measure its frequency response.…”

Graphene Pseudoreference Electrode for the Development of a Practical Paper-Based Electrochemical Heavy Metal Sensor

“…18,19 The stable working time of a commercial Ag/ AgCl PRE has been reported to only last for 30 min (for longterm monitoring), which has been attributed to potential drift. 15 Previously, carbon-based PREs such as graphitic carbon 20−23 or activated carbon 24 have shown promising applications ranging from pH sensing 25 and corrosion monitoring 26 to an ion-sensitive field effect transistor (ISFET)-based sensor, 27 as well as in battery and supercapacitor devices. 28,29 In this regard, our group has been particularly interested in the graphene pseudoreference electrode (GPRE) because the chemical inertness and exceptional resistance (with respect to mechanical stress, temperature change, and ionic interaction) of graphene shed new insights into electrochemical sensing.…”