Iridium Oxide-reduced Graphene Oxide Nanohybrid Thin Film Modified Screen-printed Electrodes as Disposable Electrochemical Paper Microfluidic pH Sensors

Summary

The study demonstrates the growth of iridium oxide-reduced graphene oxide (IrO₂-RGO) nanohybrid thin films on irregular and rough screen-printed carbon substrate through a green electrochemical synthesis, and their implementation as a pH sensor with a patterned paper-fluidic platform.

Abstract

A facile, controllable, inexpensive and green electrochemical synthesis of IrO₂-graphene nanohybrid thin films is developed to fabricate an easy-to-use integrated paper microfluidic electrochemical pH sensor for resource-limited settings. Taking advantages from both pH meters and strips, the pH sensing platform is composed of hydrophobic barrier-patterned paper micropad (µPAD) using polydimethylsiloxane (PDMS), screen-printed electrode (SPE) modified with IrO₂-graphene films and molded acrylonitrile butadiene styrene (ABS) plastic holder. Repetitive cathodic potential cycling was employed for graphene oxide (GO) reduction which can completely remove electrochemically unstable oxygenated groups and generate a 2D defect-free homogeneous graphene thin film with excellent stability and electronic properties. A uniform and smooth IrO₂ film in nanoscale grain size is anodically electrodeposited onto the graphene film, without any observable cracks. The resulting IrO₂-RGO electrode showed slightly super-Nernstian responses from pH 2-12 in Britton-Robinson (B-R) buffers with good linearity, small hysteresis, low response time and reproducibility in different buffers, as well as low sensitivities to different interfering ionic species and dissolved oxygen. A simple portable digital pH meter is fabricated, whose signal is measured with a multimeter, using high input-impedance operational amplifier and consumer batteries. The pH values measured with the portable electrochemical paper-microfluidic pH sensors were consistent with those measured using a commercial laboratory pH meter with a glass electrode.

Introduction

The determination of pH is ubiquitous in food, physiological, medicinal and environmental studies. Two most common tools for pH detection are pH strips and pH meters. Paper strips are impregnated with color-changing pH indicator molecules but the reading is sometimes limited in pH ranges, subjective and semi-quantitative with some deviations. On the other hand, a pH meter conventionally equipped with a glass electrode can measure pH accurately to the 0.01 level, and display by a digital-user interface. Lab-based pH meters not only need special care in maintenance and calibration, but also do not work well towards small sample volumes and often require a clean containe....

Protocol

1. µPAD and Apparatus Preparation

1. Engrave a 500 µm groove on the bottom plastic holder to house SPE with an ABS or compatible plastic sheet by three-dimensional (3D) milling machine and milling bit which has 1.6 mm of diameter. Hold SPE and µPAD firmly in place during testing with the holder (Figure 1A).
2. Make a stamp and a vacuum cover using synthetic resin tablet or compatible plastic sheet with convex and concave patterns, respectively, by the 3D milling machine, in order to pattern hydrophobic PDMS barriers on paper pads.
   1. Prepare a mixture of PDMS pre-polymer and cross linker at the ratio of 10:1 or ....

Results

The setup of the electrochemical IrO₂-RGO-SPE pH sensor incorporating paper microfluidics is shown in Figure 1A. The patterned paper pad with PDMS hydrophobic barriers was placed on top of the sensing area of IrO₂-RGO-SPE which located on the ABS plastic holder. The sensing zone of paper pad was carefully aligned with electrode surface. An aqueous methylene blue dye solution was used to test the patterned paper pad and as observed, samples wick into .......

Discussion

Device Setup

The pH sensor works by measuring the OCP between the working and reference electrodes, since it changes proportionally to the negative logarithm of H⁺ concentration. The measurements can be achieved both by a lab-based potentiostat such as CHI 660D and simple pH meter constructed on breadboard with reading by multimeter. Two different portable pH meters were built similarly on breadboards using two 9 V alkaline batteries, a digital multimeter, as-synthe.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Screen-printed electrodes	Zensor	TE100	3-electrode integrated
Acrylonitrile butadiene styrene (ABS)
Polydimethylsiloxane (PDMS) prepolymer and cross linker mixture	Dow-Corning Co.	Sylgard 184	10:1 mixture w/w
Whatman No. 1 filter paper	GE Healthcare Co.
3D milling system	Roland DGA Co.	iModela IM-01
PDMS stamp and vacuum cover	Roland DGA Co.	Sanmodur	Synthetic resin tablet
Hand-operated vacuum pump	Cole-Parmer Co.
Electrochemical workstation	CH Instruments	CHI 660D
LF356N operational amplifiers	Texas Instruments Inc.
INA111 high speed field-effect transistor (FET)-input instrumentation amplifier	Burr-Brown Inc.
DMM914 digital multimeter	Tektronix Inc.	70979101
From Fisher or Sigma:
Iridium tetrachloride (IrCl4)
50% (w/w) hydrogen peroxide (H2O2)
Oxalic acid dihydrate
Potassium carbonate (K2CO3)
Phosphoric acid
Acetic acid
Boric acid
Sodium hydroxide (NaOH)
Na2HPO4
NaH2HPO4