Name: iridium oxide-reduced graphene oxide nanohybrid thin film modified screen-printed electrodes as disposable electrochemical paper microfluidic ph sensors
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This work was supported by a grant from the Water Equipment and Policy (WEP) NSF Industry/University Cooperative Research Center (I/UCRC). The authors are also thankful to the Hjalmar D. and Janet W. Bruhn Fellowship and Louis and Elsa Thomsen Wisconsin Distinguished Graduate Fellowship provided to J. Y. at UW-Madison

1. &#181;PAD and Apparatus Preparation
<ol>
	<li>Engrave a 500 &#181;m 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 &#181;PAD firmly in place during testing with the holder (Figure 1A).</li>
	<li>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...

<ol>
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Chem. 84, 4147-4152 (2012).</li><li>Dossi, N., Toniolo, R., Pizzariello, A., Impellizzieri, F., Piccin, E., Bontempelli, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pencil-drawn+paper+supported+electrodes+as+simple+electrochemical+detectors+for+paper-based+fluidic+devices.">Pencil-drawn paper supported electrodes as simple electrochemical detectors for paper-based fluidic devices.</a> Electrophoresis. 34, 2085-2091 (2013).</li><li>Yang, J., Gunasekaran, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemically+reduced+graphene+oxide+sheets+for+use+in+high+performance+supercapacitors.">Electrochemically reduced graphene oxide sheets for use in high performance supercapacitors.</a> Carbon. 51, 36-44 (2013).</li><li>Yamanaka, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anodically+electrodeposited+iridium+oxide+films+(AEIROF)+from+Alkaline+Solutions+for+Electrochromic+Display+Devices.">Anodically electrodeposited iridium oxide films (AEIROF) from Alkaline Solutions for Electrochromic Display Devices.</a> Jpn. J. Appl. Phys. 28, 632-637 (1989).</li><li>Yamanaka, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+electrochemical+behavior+of+anodically+electrodeposited+iridium+oxide+films+and+the+reliability+of+transmittance+variable+cells.">The electrochemical behavior of anodically electrodeposited iridium oxide films and the reliability of transmittance variable cells.</a> Jpn. J. Appl. Phys. 30, 1285-1289 (1991).</li><li>Fog, A., Buck, R. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electronic+semiconducting+oxides+as+pH+sensors.">Electronic semiconducting oxides as pH sensors.</a> Sens. &amp; Act. 5, 137-146 (1984).</li><li>Bezbaruah, A. N., Zhang, T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+of+anodically+electrodeposited+iridium+oxide+film+pH+microelectrodes+for+microenvironmental+studies.">Fabrication of anodically electrodeposited iridium oxide film pH microelectrodes for microenvironmental studies.</a> Anal. Chem. 74, 5726-5733 (2002).</li><li>Marzouk, S. A. M., Ufer, S., Buck, R. P., Johnson, T. A., Dunlap, L. A., Cascio, W. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrodeposited+iridium+oxide+pH+electrode+for+measurement+of+extracellular+myocardial+acidosis+during+acute+ischemia.">Electrodeposited iridium oxide pH electrode for measurement of extracellular myocardial acidosis during acute ischemia.</a> Anal. Chem. 70, 5054-5061 (1998).</li><li>Prats-Alfonso, E., Abad, L., Casa&#241;-Pastor, N., Gonzalo-Ruiz, J., Baldrich, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Iridium+oxide+pH+sensor+for+biomedical+applications.+Case+urea-urease+in+real+urine+samples.">Iridium oxide pH sensor for biomedical applications. Case urea-urease in real urine samples.</a> Biosens. Bioelectron. 39, 163-169 (2013).</li><li>Bitziou, E., O'Hare, D., Patel, B. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Simultaneous+detection+of+pH+changes+and+histamine+release+from+oxyntic+glands+in+isolated+stomach.">Simultaneous detection of pH changes and histamine release from oxyntic glands in isolated stomach.</a> Anal. Chem. 80, 8733-8740 (2008).</li></ol>

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...

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 container such as a beaker to perform measurements. In spite of its sensitivity, selectivity and stability, glass electrodes suffer from acid/alkaline errors, high impedance, temperature instability and mechanical fragility1. Therefore it is advantageous to have a pH measurement system that embodies the accuracy of pH meter and the simplicity and cost aspects of pH strips.
There is always an unmet need for such tools under limited resources conditions in many developing regions where expensive lab-based equipment or commercial laboratories are unaffordable. Also, the increasing role of new easy-to-use on-site sensing platforms is pushed by such a demand for point-of-care detection. Electrochemical detection is simple, easy to miniaturize and satisfactorily sensitive, as demonstrated by the commercialized low-cost SPEs and various glucose monitoring systems on the market. As a light, flexible and disposable porous material, paper can also have various controllable characteristics, such as different pore sizes, functional groups, and wicking rates.
As paper substrate barely affects analyte diffusion and electrochemical detection2-4, combination of paper-fluidic devices and electroanalytical techniques has recently received extensive interests. An apparent advantage of such combinations is the tiny amount of sample volume used in the measurement which can potentially prevent interferences from vibration and convection during measurements. For instance, patterned microfluidic pads were applied to wick and deliver liquid samples to sensing area of SPEs for detection of heavy metal ions and glucose2,5. Similar devices using paper microfluidic electrochemiluminescence were established to accomplish NADH detection4. More recently, simple electrochemical paper microfluidic devices can be built on a glass slide with pencil electrodes6 or using enzyme paper and SPEs3.
A nanohybrid thin film material composed of IrO2 and RGO was prepared using a facile and efficient electrochemical approach. We found that on the irregular and rough SPE graphitic carbon surface, anodically electrodeposited IrO2 thin film cannot be smooth and stable without the aid of RGO. The resulting IrO2-RGO SPE was integrated into a paper microfluidic device which has patterned hydrophobic barriers for pH sensing. The assembled device showed excellent analytical performances in pH sensing with a slightly super-Nernstian behavior. The results are comparable to a conventional lab-based pH meter with glass electrodes. Lastly, cost-effective miniaturized pH meters were built on a breadboard to measure open circuit potential output signal with a digital multimeter.&#160;The measurements of the portable pH meter correlates well with those of a commercial laboratory pH meter.&#160;

<table>
	<tbody>
		<tr>
			<td>Screen-printed electrodes</td>
			<td>Zensor</td>
			<td>TE100</td>
			<td>3-electrode integrated</td>
		</tr>
		<tr>
			<td>acrylonitrile butadiene styrene (ABS)&#160;</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Polydimethylsiloxane (PDMS) prepolymer and cross linker mixture</td>
			<td>Dow-Corning Co.</td>
			<td>Sylgard 184</td>
			<td>10:1 mixture w/w</td>
		</tr>
		<tr>
			<td>Whatman No. 1 filter paper</td>
			<td>GE Healthcare co.</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>&#160;3D milling system</td>
			<td>Roland DGA Co.</td>
			<td>iModela IM-01</td>
			<td></td>
		</tr>
		<tr>
			<td>PDMS stamp and vacuum cover</td>
			<td>Roland DGA co.</td>
			<td>Sanmodur</td>
			<td>Synthetic resin tablet</td>
		</tr>
		<tr>
			<td>hand-operated vacuum pump</td>
			<td>Cole-Parmer co.</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Electrochemical workstation</td>
			<td>CH Instruments</td>
			<td>CHI 660D</td>
			<td></td>
		</tr>
		<tr>
			<td>LF356N operational amplifiers</td>
			<td>Texas Instruments Inc.</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>&#160;INA111 high speed field-effect transistor (FET)-input instrumentation amplifier</td>
			<td>Burr-Brown Inc.</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>DMM914 digital multimeter&#160;</td>
			<td>Tektronix Inc.</td>
			<td>70979101</td>
			<td></td>
		</tr>
		<tr>
			<td>From Fisher or Sigma:</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>&#160;iridium tetrachloride (IrCl4)</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>50% (w/w) hydrogen peroxide (H2O2)</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>oxalic acid dihydrate</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>potassium carbonate (K2CO3)</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>phosphoric acid</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>acetic acid&#160;</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>boric acid</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>sodium hydroxide (NaOH)</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Na2HPO4</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>NaH2PO4</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

iridium oxide-reduced graphene oxide nanohybrid thin film modified screen-printed electrodes as disposable electrochemical paper microfluidic ph sensors

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 (&#181;PAD) 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.

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 ...

Watch this Scientific Journal Video about Iridium Oxide-reduced Graphene Oxide Nanohybrid Thin Film Modified Screen-printed Electrodes as Disposable Electrochemical Paper Microfluidic pH Sensors at Jo

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

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.

A facile, controllable, inexpensive and green electrochemical synthesis of IrO2-graphene nanohybrid thin films is developed to fabricate an easy-to-use ...

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